I see a lot of younger writers struggling with the use of passed and past. I know they sound the same, but they have very different meanings.
To start with, past is a noun; passed is a verb. But to complicate things, past is also a modifier; it can be an adjective or an adverb. Let’s look at some examples.
Passed means to go by, to move beyond, some other object.
He passed her on the street.
We passed the cup from hand to hand.
She passed the test.
The usage of passed should always be as an action.
Past refers to a time and can be used as a noun:
The cataclysm happened in the distant past.
She feared her lover would discover the terrible secrets of her past life.
or an adverb, and this is usually where people get confused, because the adverb form of past means to go beyond:
He ran past.
The motorcycle was past them before they knew it was there.
The old man shuffled past the gate.
Still confused? Think of it this way; if there’s no verb in your phrase, then you are usually safe to use passed. However, if you already have the verb, then you need to use past.