[grammar 3] Auxiliary verb 2

We may (or might) as well fight as escape.
He may (or might) well be surprised at the news.
I woud rather go to bed early than sit up late at night.
I would rather not sleep.
I'm used to use PC.
Wood is used to create a house.
I need not have done the work today.
I should have gone there soon.
He may (or might) have left for Japan.
We must have completed the mission.
He cannot have mistaken.