I don’t think used instead of I don’t think so.
Don’t say:    I don’t think.
Say: I don’t think so.
I don’t think means I don’t use my brains, while I don’t think so means I am not of that opinion.

 Throw it used instead of throw it away.
Don’t say:    It’s dirty, throw it.
Say: It’s  dirty, throw it away.
Throw it means to throw a thing at someone or somewhere, such as a ball. Throw it away means to get rid of it by throwing it aside.

 Up and down used  instead of upstairs and downstairs.
Don’t say:    He’s up, he’s down.
Say: He’s upstairs, he’s downstairs.
He’s up means he’s out of bed.  He’s upstairs (downstairs) means he’s on the  upper  (lower)  floor  of  the  building.

Better used instead of had better.
Don’t say:   Better go home  at once.
Say: You’d better go home at once.
The correct phrase  is had better.  You had  better go  means It would be a good  thing for you to go.

For this used instead of for this reason.
Don’t say:    For this he wants to  leave.
Say: For this reason he wants to leave.
The  phrase for this  is  incorrect.  Say for this reason or for that reason   Also owing to that or  because of that.

Omission of the word old from age.
Don’t say:    My  sister is  fifteen years.
Say: My sister is fifteen years old.
Note:  We  can also  Say: My sister is  fifteen years of age,  or  simply,  My sister is   fifteen.

 Omission of the word or between numbers.
 Don’t say:   I’ve  only two, three friends.
Say: I’ve only two or three friends.
We must always  insert  the  conjunction  or between  numbers like  this   two or three men, five or six  pages, eight or ten days.

 Omission of the word and between numbers.
Don’t say:    Eight  thousand  thirty-seven.
Say:  Eight  thousand and thirty-seven.
Use the conjunction and to connect  hundred,  thousand,  million to a  number of tens or units.

Omission of the  noun  after an  adjective.
Don’t say:  The  unfortunate  was  shot  dead.
Say: The unfortunate man  was shot dead.
The  noun that comes after an adjective can’t be understood;  it must be expressed
Note:  Omit the noun after an  adjective only when the adjective is used  as a noun  in  the  plural   The poor envy the rich.

 The object of the verb enjoy omitted.
Don’t say:   I  enjoyed during the  holidays.
 Say: I  enjoyed myself during the holidays.
Or: I enjoyed my holidays.
Don’t follow the verb enjoy  by a  preposition.  It  must always have an  object, which may either be a reflexive pronoun or a noun.
Note: We Say: I had a good time, as this is an idiomatic expression, but we can’t  say I enjoyed my  time.  We  must  specify. I enjoyed my  time  in  Greece.

Omission of the direct object when there are two objects.
Don’t say:   I asked him for some ink, and he gave me.
Say: I asked him for some ink, and he gave me some.
Some transitive verbs, like give, bring, send, tell, buy, show, must have two expressed objects, direct and indirect  here, some is the direct object of gave .

The object of the transitive verb omitted.
Don’t say:   I  asked  her for some paper,  but she had  not.
Say:  I asked her for some paper, but she had none didn’t have any.
As a rule, every transitive verb must have an expressed object: here, none (equivalent to not any) is the object of had.

 Omission of the personal pronoun  after a  quotation.
Don’t say:   ‘I’m  learning English,’  said.
Say:  ‘I’m  learning English,’  he said.
After a quotation, express the personal pronoun as the subject of the reporting verb.

 Omission of the pronoun subject from the  principal clause.
Don’t say:   When he  saw the teacher, stood up.
Say: When he saw the teacher, he stood up.
In a sentence beginning with an adverbial clause, express the personal pronoun as the subject of the main clause.

Omission of it as subject of an impersonal verb.
Don’t say:    Is very hot in the Sudan.
Say: It’s very hot in the  Sudan.
Use the pronoun it as the subject of an impersonal verb

Omission of the personal pronoun before the infinitive.
Don’t say:   I want to  tell me the truth.
Say: I want you to tell me the truth.
Express the subject of the infinitive after verbs like want, like, wish, etc , if it is different from that of the main verb.

 Omission of the demonstrative pronoun one.
Don’t say:   This is the only that I like.
Say: This is the only one that I like.
Use the demonstrative pronoun one (plural ones) in place of a noun mentioned before.

Omission of else after everybody, etc.
Don’t say:   She is stronger than everybody.
Say: She is stronger than everybody else.
Use the word else in making a comparison between one person or thing and all others of the same kind after everybody, anybody, anything, etc.

Omission of before in comparisons.
Don’t say:  I’d never seen such a thing.
Say: I’d never seen such a thing before.
Don’t leave out the word before in making a comparison between one thing and all others of the same kind.

 Omission of other after a comparative.
Don’t say:   Homer was greater than all the Greek poets.
Say: Homer was greater than all the other Greek poets.
Since Homer was a Greek poet, the first sentence makes him greater than himself, which is illogical.

Omission of how after the verb to know.
Don’t say:   She knows to play the piano.
Say: She knows how to play the piano.
After the verb to know the adverb how always comes before an infinitive.

Omission of there as an introductory word.
Don’t say:   Once lived a great king.
Say: Once there/There once lived a great king.
Use the adverb there to introduce the subject of a sentence in which the verb stands before the subject.

Omission of the preposition after the infinitive.
Don’t say:  They’ve no houses to live.
Say: They’ve no houses to live in.
The infinitive of an intransitive verb (like live, etc): it must have a preposition after it.
Have another look at …
Present   tense I am’m, you are’re, he (she,  it)  is’s; We, you, they are’re.
Past Tense   I  was,  you  were,  he  (she,  it)  was; We, you, they were.
Future Tense      I, you, he (she, it) will’ll be; We, you, they will’ll be.
Present Perfect I, you, have’ve been, he (she, it) has’s been; We, you, they have’ve been.
Past Perfect   I, you, he (she, it) had’d been; We, you, they had’d been.
Future Perfect I, you, he (she, it) will’ll have been; We, you, they will’ll have been.
Uses of the verb TO BE as auxiliary
Use the verb to be:
1. With the Present Participle to form the Continuous Tenses.
To be + Present Participle
Example: The sun was shining in the sky.
2. With the Past Participle to form the Passive Form. To be + Past Participle
Example: The letter was written by John.

Omission of the preposition indicating time.
Don’t say:  I was born the third of December.
Say: I was born on the third of December.
As a rule, don’t use a noun without a preposition to show the time of some action.
Note: Don’t use a preposition with last year, next year, some day, one day, this afternoon, etc.

 Omission of auxiliary do when do is a principal verb.
Don’t say:  Do pupils their work carefully?
Say: Do pupils do their work carefully?
In the correct form of the sentence, the first do means nothing on its own and only helps to make the question. The second do is the principal verb of the sentence, and has the meaning of perform.

Omission of the auxiliary do from questions.
Don’t say:  You understand the problem? He understands the problem? She understood the problem?
Say: Do you understand the problem? Does he understand the problem? Did she understand the problem?
Place the auxiliary verb do (does, did) before the subject to ask questions in the simple present and simple past tenses.
Note: Don’t use the auxiliary do with modal verbs, like can, may, must: Can you meet me tomorrow?

Omission of the verb to be from the passive.
Don’t say:   Charles Dickens born in 1812.
Say: Charles Dickens was born in 1812.
Form the passive form by using the verb to be, combined with the past participle of the verb required (to be + past participle).
Have another look at…
Indefinite article – Use the indefinite article:
1.  Before every common noun in the singular, if it isn’t preceded  by  the  or some  word such  as   this,  that,  my, his:   I bought a new book  (not: I bought new book).
2. Before the  words  hundred   and  thousand:  A   hundred soldiers  were  in   the  camp.
3. After the verb to be when a countable noun in the singular follows:  Mary’s father   is  a  lawyer.
4. In certain phrases: to make a noise, a mistake, a fortune, an impression; to have a headache, a pain, a cold, a cough.
Don’t use the indefinite article:
1. Before singular nouns that aren’t used in the plural, such  as   advice,   information,   work,   furniture,  bread Example: He gave me good advice (not: a good advice).
2.After the phrase kind of or sort of: What kind of pen do  you   want? a, an or one
Many languages use the numeral one instead of the indefinite article a or an. This is not so in English. One man went into one shop ought to be A man went into a shop. One is used only when the number is emphatic:   One swallow does not make a summer.

Omission of the before the word cinema, etc.
Don’t say:   On Saturday I go to cinema.
Say: On Saturday I go to the cinema.
Use definite article before the words cinema, theatre, concert, etc

Omission of the before names of musical instruments.
Don’t say: 
  I play violin, but not piano.
Say: I play the violin, but not the piano.
Use the definite article before the names of musical instruments.

Omission of the before names of nationalities.
Don’t say:   English are fond of sports.
Say: The English are fond of sports.
Place the definite article before the names of nationalities, describing a people collectively the British, the French, the Dutch, the Swiss, the Chinese, the Sudanese, etc.

Omission of a or an from make a noise, etc.
Don’t say:   I told them not to make noise.
Say: I told them not to make a noise.
Note: Also to make a mistake, to make a fortune, to make a will, to make an impression, to make an experiment, to make an attempt.

Omission of a or one before hundred, etc….
Don’t say:   Hundred years make a century.
Say: A hundred years make a century.
Or: One hundred years make a century.
Use the indefinite article a or the numeral one before hundred and thousand. See also Section 527.

Omission of a or an after the word half.
Don’t say:   He  drank  half glass  of milk.
Say:  He  drank half a glass of milk.
Note: Half a glass (an hour, a day, a mile, etc.) is the shortened form of half of a glass (of an hour, of a day, of a mile, etc.).

Omission of a or an after the verb to be.
Don’t say:    I’m not  teacher,  I’m student.
Say: I’m not a  teacher, I’m a student.
Use the indefinite article a or an to express a singular noun-complement of the verb to be  There’s an animal in there. It’s a mouse.

 Omission of the article before a countable noun in the singular.
Don’t say:  I’ve  no  money  to  buy  car.
Say: I’ve no money to buy a  car.
As a rule, use either the or a or an before a countable noun in the singular.

 The possessive ending omitted.
Don’t say:   A  hen’s  egg  is  different from  a pigeon.
Say: A hen’s egg  is different from a pigeon’s.
If the first noun in a comparison is in the possessive case, the second must also be in the possessive: My mother’s nose is bigger than my father’s.

The – s , -es or -ies of the plural form omitted.
Don’t say:  I paid six  pound  for the book.
  Say: I paid six pounds for the book.
Take care not to leave out the -s, -es or -ies of the plural number.
Note:  the following nouns have irregular plurals: man, men; women, women; child, children; ox, oxen; foot, feet; tooth, teeth; goose, geese; mouse, mice.

 The -d or -ed of the past tense omitted.
Don’t say:   I receive a letter yesterday.
Say: I received a letter yesterday.
Take care not to leave out the -d or -ed from the past tense of regular verbs. When speaking, pronounce the ending of the past tense clearly.

Have another look at …
Third person singular, simple present
1.With the pronouns he, she, it, or any singular noun, the verb in the present tense takes a special ending, -s, -es or-ies: he works, it catches,  the sun rises, she worries.
2. When the first person of the verb ends in s, x,  ch, sh, or o, the third person singular takes -es: I watch, I finish, I fix, I go, & he watches   he finishes  he fixes    he goes
3. When the first person of the verb ends in y with a consonant before it, form the third person  singular by changing y into ies:
I carry        I study        I fly
he carries    he studies   he  flies
Note: If there is a vowel before the y, we only add s for the third person singular: he plays, he enjoys, he obeys.
4. A few verbs are irregular in the third person singular:
I am             I have          he is           he has
5. Modal verbs such as will, can,  may,  must,  and  ought do NOT change their form in the third person singular:
I will           I can           I may      I must
he will        he can         he may    he  must
Remember: The third person singular of verbs in the present tense takes -s, -es or -ies.

Using don’t instead of doesn’t.
Don’t say:   He don’t care what he  says.
Say: He doesn’t care what he says.
Use don’t (= do not) with I, we, you, they, and with plural nouns. Use doesn’t (= does not) with he, she, it, and with singular nouns.

The -s or -es of the third person singular omitted.
 Don’t say:   He speak  English  very well.
Say: He speaks English very well.
Take great care not to leave out the -s or  es from the present tense, when the subject is he, she, it, or a noun in the singular.