Bullocks passing a cottage.

These two frequently confused words just popped into my mind as I was writing on Facebook. So, here come some example sentences to make things clear.

Some of the bullocks have already passed (verb) the house. As they walked past (adverb) the window, some of them looked in.  Then they walked along the lane, past the phone box, and on to their field.
I was 18 when I passed my driving test. (verb)
I passed through customs control very quickly with my biometric passport. (verb)
His past crimes have left him difficult to employ. (adjective)
He has a murky past – we know little about him. (noun)
The car sped past into the night. (adverb or preposition)

In summary

Passed is the past tense of to pass and is always a verb.
Past can be an adverb or preposition (he walked past the door without stopping).
Past can be an adjective – the past economic crises have left the economy weak.
Past can be a noun – his criminal past haunts him still.

I hope this clears up the confusion. If not, please leave a comment and I’ll get back to you.