Use past to refer to the time that came before now or to refer to beyond something in distance. His ten years of working for a corporation with a bad reputation is in the past (the time before now). Their past (the time before now) poor treatment of employees has come back to haunt them. She drove past (beyond something in the distance) the building.
Use passed as the past tense of the verb to pass. I passed the building. She passed on a job offer from the company because of its poor reputation. He passed judgment on the case. Because of her youthful looks, she passed for someone much younger.
Whenever you can use pass in the present tense, use passed for the past tense. For the above sentences, we’d have I pass the building, She passes on a job offer, He passes judgment, She can pass for someone younger.
Sherry
Paul’s book–Literally, the Best Language Book Ever;
Everything Language and Grammar 