Why not pomoć? I thought infinitives ended in ć.
There are some that don't, for example
pomóc - to help,
móc - to be able,
biec - to run,
piec - to bake
The hint for mamie says Locative. Is it not Dative?
Here it is Dative. but it can be locative. The hints are crazy.
mojej also has hint for Genitive and Locative but not for Dative.
Muszę is must, isn't it?
Yes, but English "have to" also corresponds to it.
Yes, but better to think of it as primarily "have to" as "nie musieć" means "don't have to". And in any case "have to" is used more generally than "must".
I never though about how there is no "do not must." It is neat what you learn about your own language while learning another's.
But there is "you must not" ("you can't") = "nie wolno" ("nie można")
Well, in some cases "you must not" and "you can't" may work synonymously, but in general, "you must not" means "you're not allowed to".
I have suggested "mummy" in addition to "mum" for those of us who are not American.