The Difference Between "Must" And "Have To"
Both the modal verb must and the expression have to are used to express obligation in the present, but they have subtle differences. Here below you can find some information that helps you to remember when to use one form rather than the other.
- Must is used for internal obligation while have to is used for external obligation:
I MUST remember to call my parents tomorrow → the obligation comes from inside factors (e.g., my resolution).
I HAVE TO remember to take my pills → the obligation comes from outside factors (e.g., the Doctor). - Must is used for strong recommendation while have to is used for a somewhat weaker recommendation:
This book is required by the exam rules, you MUST read it → you might have no other choices than actually reading the book.
This book is really good, you HAVE TO read it → the recommendation is to read the book, but you might not need to read the book. - Must is used to reflect supposition, or deductive reasoning:
It’s very late. They MUST be sleeping. → Since it is very late, I conclude that the people might be sleeping.
- Have to is used instead of Must in past verbal forms, since the modal verb Must does not have a past form (e.g., "musted" does not exist):
Yesterday I HAD TO call my parents, but I actually forgot about it, I MUST call them today. → In this case Have to is simply used to reflect MUST, but in the past.