Relatively speaking, if you use past or passed depends on whether you are using a verb or a noun.

If wanting to show that something has occurred, use the verb passed: The motorcycle passed, and everything quieted once more. (What occurred is the motorcycle went by.).

If wanting to show what you’re referring to, use the noun past: In the past when the road was gravel, motorcycles never went on it. (You’re referring to a specific thing, the past.)

A lot of writers get mixed up with the whole space-time continuum thing, thinking that “past” means something once was in the state of occurring. Leave relativity to Einstein, though, and think noun-verb when deciding which word to use.

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My name is Rob Bignell. I’m an affordable, professional editor who runs Inventing Reality Editing Service, which meets the manuscript needs of writers both new and published. I also offer a variety of self-publishing services. During the past decade, I’ve helped more than 300 novelists and nonfiction authors obtain their publishing dreams at reasonable prices. I’m also the author of the 7 Minutes a Day… writing guidebooks, four nonfiction hiking guidebook series, and the literary novel Windmill. Several of my short stories in the literary and science fiction genres also have been published.