Must... Mustn't ... Don't need to....

Question 1

I must (do something) =

Answer 1

I need to do it

…I’m very hungry. I must eat something.It’s a fantastic film.
…You must see it

Question 2

I mustn’t (do something) =

Answer 2

it is necessary not to do it, it is the wrong thing to do

…I must go. I mustn’t be late.
…I mustn’t forget to phone Chris (= I must remember to phone her)

Question 3

For the past (yesterday / last week etc.)

Answer 3

we use had to … (not must)

…I was very hungry. I had to eat something. (not I must eat)
…We had to walk home last night. There were no buses. (not We must walk)