Supposed to

Supposed to is a descriptive expression derived from the passive voice, so the past participle supposed is always the correct form when it is used to mean “permitted,” “required,” or “expected”:

was suppose to be home by 12
was supposed to be home by 12

not suppose to do that
not supposed to do that

The mistake of leaving off the participial suffix is very common and easy to understand; we rarely pronounce both the ending d and the following to clearly and distinctly, so they naturally blend together into one sound as if the suffix were not present.