We must do the same thing again.
We mustn't do the same thing again.
You may sit here.
You may not sit here.
We can meet you later.
We can't meet you later.
We should tell her about it.
We shouldn't tell her about it.
She should sit near the window.
She shouldn't sit near the window.
I can understand him easily.
I can't understand him easily.
She can play the piano well.
She can't play the piano well.
We must wait here.
We mustn't wait here.
They can go by plane.
They can't go by plane.
He may leave on Wednesday.
He may not leave on Wednesday.