It's a bit confused to me. What's the difference?
Must - Have to - Got to
"Have to" we are using when we are talking about the activity which must categorically be made, for example-"Everyone, in the certain time, will have to die." We are using "Must" when we are talking about some activity, which is an initiator of the continuation of events-for example-"I must sleep each night in order to feel well rested". "Got to" we are using when we are speaking about the activity ,which we carried out ,which cost us of sure efforts, and which the workmanship is ours, for bigger or smaller, a success. For example- "He let me I got to know his curriculum vitae."
Must is used for an external obligation while Have is for an internal obligation Example: 1.I have to revise my lessons tonight( Internal obligation) 2.Father to his son: You must revise your lessons tonight.
P.S: got to is an informal form of have got to
Have got to is used interchageably with Have to by The british.
must, have to and got to all refer to obligation/permission.
in meaning, the main difference between must and have to is where the obligation comes from. in case of have to, you are forced to do something that you might not like to do but you have to do them (rules, laws are typically like this). in case of must, the obligation comes from the 'inside', so you're doing something because it's in your interest. or we use it for others when we want to arouse their interest in something (e.g. Son, you must do your homework).
according to my studies, the strength of the obligation doesn't matter in these cases just the source of it. additionally, must can also mean 'it's very certain that'. e.g. She finishes work at 6, so she must be at home after 7.
in negative sentences: don't have to / doesn't have to means that something is not necessary. similar to don't need to / doesn't need to. mustn't means that something is prohibited so its negative meaning can be very misleading because it has nothing to do with obligation.
got to is very informal and generally spoken english, it's short for have got to or have to. source: http://www.ldoceonline.com/dictionary/gotta
if you aren't sure about the differences, use need to.