Error (Auxiliary & Modal Verb)

Auxiliary verbs (Helping verb)

Modal auxiliaries are can, could, may, might, must, 

shall, should, will, and would.

I used to take a nap in the afternoon but now I don't.

I am used to eating spicy food now.

A: Could I use your phone?

B: Yes, you can.

A: Must we climb over the fence?

B: No, you needn't.

A: Need she leave the room?

B: Yes, she must.

You should have bought the lottery ticket yesterday. 

Peter seems unhappy. He must have quarrelled with his girlfriend.

Shall I start the meeting?

It is moving. It can't be a rope. It must be a snake.

A: Can somebody mop the floor?

B: I will.