must   

All  students must wear masks in school.

Cars must not park in front of theentrance (= it is not allowed).

Must Iwear masks in school?

Yes, you must. 

No, you needn't   /No, you don't have to.

You must be hungry after all thatwalking.

He must have known (=surely he knew) what she wanted.  

Harry porter is really afun to read. You simply must read it.

I’ve ordereda big pizaa – that should beenough.

That must be enough

 I'llphone May– she should behome by now.

She must be home bynow.

Obligation

You shouldn't saythings like that to Anny

You mustn't saydirty words in public.

 must + have + 过去分词  should + have + 过去分词

She already knew ofit. He must have called her this morning

I should have phoned Ed this morning, but I forgot.