MODAL VERBS OF OBLIGATION

MUST

only present form

followed by infinitive without to

Ex. I MUST do my homework

PERSONAL obligation

You MUST see the doctor

HAVE TO

-s in the 3rd person singular

Do-Does for questions

All tenses

Rules, laws, coming from OUTSIDE

SHOULD/SHOULDN'T

ADVICE

We should leave early

We shouldn't stay up until late