Passed vs Past

Does anyone else have problems with knowing when to use the correct one?
Going through my ms, i have been struck with the knowledge that i am confused by it, so much so that i change them from one to the other and back again, but still don't think i know!

I try to remember that:
* Passed is the past participle of to pass

And,
* Past refers to time. (But that changes when someone walks past a shop!)

This article is very good, but i still don't think i have a handle on it.

Can anyone help out? How do you know which one to use?