Have To

"Have to" is used to express certainty, necessity, and obligation.
Examples:

·       This answer has to be correct. certainty

·       The soup has to be stirred continuously to prevent burning. necessity

·       They have to leave early. obligation

USING "HAVE TO" IN PRESENT, PAST, AND FUTURE

"Have to" behaves quite irregularly in the past and the future. Study the chart below to learn how "have to" behaves in different contexts.

Use	Positive Forms 1. = Present   2. = Past   3. = Future	Negative Forms 1. = Present   2. = Past   3. = Future	You can  also use:
have to certainty	1. That has to be Jerry. They said he was tall with bright red hair. 2. That has to have been the right restaurant. There were no other restaurants on the street. 3. NONE	1. SHIFT TO "MUST"  That must not be Jerry. They said he has blond hair, not red hair. 2. SHIFT TO "MUST"  That must not have been the right restaurant. I guess there was another one around there somewhere. 3. NONE	must,  have got to
have to necessity	1. She has to read four books for this literature class. 2. She had to finish the first book before the midterm. 3. She will have to finish the other books before the final exam.	1. She doesn't have to read "Grapes of Wrath." It's optional reading for extra credit. 2. She didn't have to write a critique of "The Scarlet Letter." She had to give a presentation to her class. 3. She won't have to take any other literature classes. American Literature 101 is the only required course.	must

REMEMBER: "Do not have to" vs. "Must not"  "Do not have to" suggests that someone is not required to do something. "Must not" suggests that you are prohibited from doing something. Examples: ·       You must not eat that. It is forbidden, it is not allowed. ·       You don't have to eat that. You can if you want to, but it is not necessary.