Must do なければなりません

This is a mouthful!  But it is so useful. Learn it well

TO MAKE IT:  plain negative form - i +  ければ なりません kereba narimasen

ピーマン を 食べなければ なりません。 
[Would be 食べなfor the plain negative form ]
pi-man o tabenakereba narimasen.
(I) must eat green peppers.  (Many Japanese children don't like green peppers)

Say that 5 times fast with your mouth full!

Perhaps the most useful usage is:

~しなければ なりません
The shi is from suru (to do)

勉強 しなければ なりませ