Modal Verbs - English Grammar

“must“ is only possible in the present tense, The replacement in all other English tenses is “(to) have to + base form”.

• They have to join the group.
• We had to pay taxes.
• I have had to travel to Tokyo.
• You will have to look for a new job.

In almost all cases "must" und "have to" are interchangeable in the present tense.

must

• We must do this today.

have to

• Mr. Doodley says you have to pack your things now.

===Negation===:

• must - need not/do not have to/does not have to
• have to/has to - need not/do not have to/does not have to
• had to - did not have to
• will have to - will not/won't have to
• have had to - haven't had to
• has had to - hasn't had to

I must finish this task today, but I do not need to hurry.
Mark has to leave early tomorrow, but he need not to/doesn't have to have breakfast.
We had to leave the bus, but we didn't have to push it out of the snow.
You will have to draw, but you won't have to present your work.
Tim has had to pay for the dinner, but he hasn't had to pay for the hotel.
I have had to dive for the keys, but I haven't had to pay for losing it.