This is another one of those sets of words that I don’t quite understand why there is all the mix-up.
Use passed if you are using a form of the verb “to pass.” If you can write the sentence in the present tense using pass, you want to use passed for the past tense.
The teacher learned that none of the students passed the test.
The teacher suspected that none of the students would pass the test. See?
Amy passed the Brussels sprouts.
After a long illness, the woman passed away.
Use past when you are referring to a time or a distance.
The team performed better during past seasons than they are now.
The girl walked her dog past the school.
In the past, people used more horses than automobiles.
So, to recap:
People aren’t past away; they passed away.
You don’t walk passed the library; you walk past the library.