Passed is primarily a past-tense verb (he passed the threshold) though can appear in a few noun phrases (passed ball and passed pawn); past can be an adjective (in the past ten years), an adverb (I ran past him), a preposition (it’s past that store), or a noun (it was in the past). In most cases, the difference is obvious, though I have noticed an unfortunate preponderance specifically for misusing past as a verb, which it cannot be.