1. After Auxiliaries / Modals:
We must go to Rome in September.
They could come with us.
2. After the auxiliary DO:
I don't think so.
3. After the following expressions:
You’d better finish your dinner.
I would rather do it now if you don't mind.
I would sooner run the marathon that watch this play.
Why not ask your mother first?
WHY SHOULD WE?
Why should we go to the cinema tonight?
4. After verbs of perception + object (action has finished):
They felt the rain fall on their faces.
I heard John come home.
5. After LET + object:
Let me go to the fun fair, please.
Let’s have a quick drink.
6. After MAKE + object:
They made her study hard.