You’ve already guessed, I’m sure, because you’re smart people. Here’s another Homophone Hell pairing: past and passed. One’s a modifier or preposition or noun, the other a verb form. And as I was reminded late yesterday, they’re evil for some people. Let’s see if I can help.
Past can be a modifier, a preposition, or a noun. As a modifier, it can denote a time (“the past year,” where it’s an adjective because it modifies a noun) or a position of a verb (“a robin flew past the window,” an adverbial use telling us “where” as part of the prepositional phrase “past the window” modifying “flew”). As a preposition, it also denotes a position, but explains a time or place (“the shadows reached past the fence to the outer edge of the yard” [there’s that adverbial use again, telling “where”] or “be ready at half past eight”).
Passed is the past tense of the verb to pass. (Note “past” in “past tense” — an adjective use.) “She passed her classes with B’s and C’s this term.” “The car passed that semi illegally.” “He passed away last year from complications caused by an infection.”
I don’t have a handy, brief, catchy mnemonic, but I will leave you with this:
She was so busy writing about her past, the dinner hour passed her by.