Form
Subject + must / have to / has to + Verb
I must finish this today.
She has to wear a uniform at work.
Meanings
Strong Obligation
We use must and have to to say something is 100% necessary. There is a small difference:
must: Usually for personal, internal obligation. The speaker feels it is necessary.
I must remember to call my mother. (This is my own feeling.)
have to: Usually for external obligation, like a rule or a law.
In the UK, you have to drive on the left. (This is the law.)
In everyday speech, many native speakers use have to for both.