NCE L17:"must" versus "have (got) to"

  There are two way in "must".

  First,it says:this is your necessity,you have to face it.at this meaning,it has no PAST tense and FUTURE tense itself.

  PAST TENSE:I had to do it.

  FUTURE TENSE:I will have to do it.

  Second,it says:this is your obligation.This is a deduction.at this meaning,it can express a past action: (modal + perfect inifitive) He must have done it.

  Another point:

  I have to do it.(no contraction)

  I've got to do it.(the contraction is always used)