It isn't necessary for her to phone me back.
She needn't phone me back / She doesn't have to phone me back / She doesn't need to phone me back.
If I were you I would tell him the truth.
You should tell him the truth / You ought to tell him the truth / You'd better tell him the truth.
I'm sure Mulder didn't lock the door. He never does.
He can't have locked the door.
It was a mistake to sell the house. I wish I hadn't.
I shouldn't have sold the house.
Ann finished the project on her own although it wasn't necessary to do so.
Ann needn't have finished the project on her own.
Perhaps John knew about the trip, but I'm not sure.
John might have known about the trip.
Perhaps she was at work when the earthquake struck.
She may have been at work when the earthquake struck. / She could have been at work when the earthquake struck.
I'm sure John locked the office before leaving. He always does.
John must have locked the office before leaving.
I think Nan is probably at home because the windows are open.
Mary must be at home because the windows are open.
Perhaps we'll go swimming tomorrow.
We may go swimming tomorrow. / We might go swimming tomorrow.