The word pass has more than two dozen distinct meanings as a verb, another dozen as a noun, and perhaps that many again as part of a phrasal verb or idiom. For our purpose here, we will try to help determine when passed should be used instead of past.
The past tense and past participle of pass is passed:
I passed (or have passed) the test. The days passed slowly.
Past is a noun (forget the past), an adjective (in past times), an adverb (drove past), and a preposition (past bedtime).
Example: Passed is the past tense of pass.