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 食べない 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 し shi is from suru (to do)

勉強　しなければ　なりません。
benkyou shinakereba narimasen.
(I) must (have to) study.

しなければ　なりません。
shinakereba narimasen.
(I) must (have to) do (it).