Поговорим по-английски!

vladirys:
Sorry, that is not the way you write it:
If must be cold outside, if the water is frozen... ( or "froze up" or
" if the is frost on the grass or the walkway" )

I'm not sure what you mean...it doesn't make any sense to me.

-mustn't is the reduced form of "must not"; however, it is not shortened very often because it sounds awkward (and like vladirys said, uneducated maybe).