Incorrect: He told her that he will come.
Correct: He told her that he would come.

Reason
When the principal verb is in the past tense the verb in the subordinate clause should also be in the past tense. When the principal verb is in the present tense, the verb in the subordinate clause can be in any tense.

Incorrect: He told me that honesty was the best policy.
Correct: He told me that honesty is the best policy.
Incorrect: Teacher said that the earth revolved around the sun.
Correct: Teacher said that the earth revolves around the sun.

Reason
To talk about general truths, we always use the present tense.

Incorrect: The cashier-cum-accountant have come.
Correct: The cashier-cum-accountant has come.

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

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

Reason
Two nouns connected by and are followed by a plural verb.

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

Reason
To talk about ability we use can, not may.

Incorrect: Tell me why are you abusing him.
Correct: Tell me why you are abusing him.
Incorrect: I wonder why don’t you listen to me.
Correct: I wonder why you don’t listen to me.

Reason
In indirect questions we put the auxiliary verb after the noun. Note that we do not use question mark in indirect questions.

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

Reason
When the noun that precedes as well as is in the singular, the verb should also be in the singular.

Incorrect: I am ill for two weeks.
Correct: I have been ill for two weeks.
Incorrect: I am waiting since morning.
Correct: I have been waiting since morning.

Reason
We use perfect continuous tenses with the prepositions since and for. Here the error lies in using the present continuous instead of the present perfect continuous.

Incorrect: The ship drowned.
Correct: The ship sank.
Incorrect: The passengers aboard the ship sank.
Correct: The passengers aboard the ship were drowned.

Reason
We use drown with people and other animate objects. We use sink with inanimate objects like ship and boat.