Incorrect: My hairs are grey.

Correct: My hair is grey.

Hair is an uncountable noun in English and as such it does not have a plural form.

Incorrect: I availed of this opportunity.

Correct: I availed myself of this opportunity.

Incorrect: I enjoyed during the holidays.

Correct: I enjoyed myself during the holidays.

To talk about having a good time, we use enjoy yourself, enjoy myself etc.

Incorrect: He is guilty, isn’t it?

Correct: He is guilty, isn’t he?

Incorrect: The gold is a precious metal.

Correct: Gold is a precious metal.

Material nouns are used without articles.

Incorrect: It is raining for four hours.

Correct: It has been raining for four hours.

To talk about an event that started in the past and has been continuing up to the present, we use a present perfect continuous tense.

Incorrect: Work hard lest you may fail.

Correct: Work hard lest you should fail.

The only auxiliary verb that can be used after lest is should.

Incorrect: Please describe us your wife.

Correct: Please describe your wife to us.

Incorrect: Could you suggest me a good dentist?

Correct: Could you suggest a good dentist to me?

The verbs explain, suggest and describe cannot be used with the structure indirect object + direct object.

Incorrect: He is cleverest of the four brothers.

Correct: He is the cleverest of the four brothers.

We use the article the with superlative adjectives.

Incorrect: Choose the best of the two options.

Correct: Choose the better of the two options.

When a group has just two members, we use the comparative instead of the superlative.