Incorrect: He told me that honesty was the best policy.
Correct: He told me that honesty is the best policy.

Explanation

The sentence ‘He told me that honesty was the best policy’ is grammatically correct. But when the subordinate clause expresses a universal truth we prefer using a present tense.

Incorrect: The cashier and accountant have come.
Correct: The cashier and accountant has come.

Explanation

Here the nouns cashier and accountant refer to the same person. Therefore, we use a singular verb.

Incorrect: The cashier and the accountant has come.
Correct: The cashier and the accountant have come.

Explanation

Here the nouns cashier and accountant refer to two people. Therefore we use a plural verb. Note the use of the articles before the nouns when they refer to different people.

Incorrect: I am so weak that I may not work.
Correct: I am so weak that I cannot work.

Explanation

To talk about ability or inability we use can, not may.

Incorrect: Tell me why are you abusing him.
Correct: Tell me why you are abusing him.

Explanation

We do not use the inverted word order (verb before subject) in an indirect question.

Incorrect: Alice as well as her sisters are intelligent.
Correct: Alice as well as her sisters is intelligent.

Explanation

When the noun that comes before as well as is in the singular number, the verb will also be in the singular. Similarly, when the noun that precedes as well as is in the plural number, the verb will also be in the plural number.

Incorrect: You will leave this place at once.
Correct: You shall leave this place at once.

Explanation

In commands and orders we use c with the second person.
Incorrect: We shall not accept defeat.
Correct: We will not accept defeat.

Explanation
To show determination we use will with first person pronouns.

Incorrect: Neither he came nor he wrote.
Correct: Neither did he come nor did he write.

Incorrect: Seldom I go to the hills.
Correct: Seldom do I go to the hills.

Incorrect: Never I have seen such a mess.
Correct: Never have I seen such a mess.

Explanation
We use inverted word order in sentences beginning with a negative word.