Must / Mustn't / Needn't

These modal verbs talk about necessity, prohibition and things that are not necessary. After all of them, use the basic form of the verb.

→ Use must to express that something is necessary, or strongly recommended:

I must leave now. = It is necessary that I leave now.
The bus leaves in two minutes. We must hurry.
This film is amazing! You must see it.

→ Use mustn't for what's forbidden, for things you can't do:

You mustn't touch the snake. = Don't do it. It is dangerous.
You mustn't forget to send the letter. = It is very important.
You mustn't park here. = It is forbidden.

→ Use needn't for what's not necessary:

You needn't hurry. = It isn't necessary
I already have everything. I needn't go shopping. = I don't have to go.

Keep in mind that mustn't and needn't have opposite meanings:

You mustn't tell anybody.
→ Don't tell anyone. It is forbidden.
You needn't tell anybody.
→ You don't have to tell anyone, but you can if you want.

Quick reference

must + verb
→ necessity / strong recommendation
mustn't + verb
→ don't do it (forbidden)
needn't + verb
→ don't have to (not necessary)
Remember: after all three forms, use the basic verb.