Passed is the past tense of pass.  How is that for confusing?

The point here is that passed is a verb and past is not.

The train passed the station. (verb)
It was hotter this summer than in past years. (adjective)
The clouds moved smoothly past the mountain peak. (adverb)
It is almost half past two and I haven’t had lunch. (preposition)
The past should teach us how to behave in the present. (noun)

When using passed, make sure that something did the passing. Otherwise use past.