green_grrl has done a marvelously thorough job detailing the different uses of past and passed
here. Let’s just hit a few of the high points for a refresher, with some help from the characters of Pirates of the Caribbean.
Several definitions of past talk about a place in time before the present. Whenever you’re describing actions that took place at some time before now, you’ll want to use past (as an adjective describing when).
“Well,” Jack said, “if you want the truth of the matter, dear William here was a pirate… but of course that was many years past,” he added hurriedly, seeing Will’s glare.
You can also use past as a noun.
“There comes a time,” Jack said, eyeing Barbossa warily, “when a man’s past, however well he believes it sunk, comes back to haunt him.”
One other common use of past is when you’re using it as an adverb or preposition, usually in the sense of “going beyond,” whether in position or in time.
“To reach the World’s End, you must go past everything you ever knew… and then go a little further,” Barbossa intoned.
Passed is the past tense form of the verb “to pass,” which has many definitions. Some of the more common ones are “to go by,” “to go or allow through” and “to depart or elapse, as in time.”
“We’ve just passed icebergs the size of mountains,” Elizabeth said, her tone cutting, “and you’re asking if I’m cold?”
“Once you’ve passed through World’s End,” Will said, reaching for the empty bottle Jack held, “everything changes. Well, nearly everything.”
“How much time passed while I was… there?” Jack asked, wiggling his fingers over one shoulder.
It’s easy to confuse the two words, given that they sound so much the same. However, one way to keep them straight is to consider whether you are describing an action or if you’re using the word to describe time. If an action is meant, you’ll want passed. If you’re describing a time before the now, it’s past.
Sources:
The Free Dictionary
here and
herePassed at Dictionary.com
Passed-vs-past at Daily Writing Tips