Necessity can be expressed using modals.
Must and have to are used to show necessity and obligation. You can also use have got to but it should only be used in casual situations.
Must and have to show that something is necessary, or required. They also show obligation. Notice how must is stronger than have to. Must could also be used to show that something is also urgent.
Have to can also be used to show that no other options are available.
Have got to is also used like have to or must. It should only be used when speaking in casual situations. Some times have is omitted. Gotta is an informal contraction used when speaking
The past form for have to, have got to, and must is had to.
Will have to is used for the future.