One is always a verb form, the other can be almost anything.
"Passed" is always a verb: the past tense of "pass". "She passed the exam" = she succeeded. "Time passed quickly" = time went by fast. Never used as an adjective or preposition.
She passed the exam with a high score.
The car passed me on the highway.
We have passed the halfway point.
"Past" can be a noun (the past), adjective (previous), preposition (beyond), or adverb. "In the past" = previously. "Walk past the store" = go by the store. Never a verb.
In the past, people didn't have smartphones.
The school is just past the traffic light.
I've been working hard for the past few weeks.
Common mistake
"She past the salt" and "He walked passed the shop" are the two most common errors: "passed" is always the verb (she passed, he has passed); "past" is everything else (noun, adjective, preposition, adverb).
Trick to remember it
"Passed" is always a verb, if you can substitute another verb (gave, approved, overtook), use "passed." For everything else (noun, adjective, preposition), use "past."
Quick rule
Did you know?
PAST is one of the most grammatically versatile words in English: it can be a noun, adjective, preposition, and adverb, four different grammatical categories for the same written form. PASSED, by contrast, has exactly one job: to be the past tense and past participle of PASS. This radical asymmetry explains the error: speakers confuse the two words because "passed" and "past" sound the same and the brain does not always distinguish grammatical function in speech. In spoken English, no one makes the error, it only appears in writing.
Exercise 1 · Pick the right one
She _____ her driving test on the first attempt. She was so relieved.
Exercise 2 · Pick the right one
The café is just _____ the post office. You can't miss it.
Exercise 3 · Form the word
'Than' is used in comparisons; 'then' indicates time, sequence, or logical consequence.
One means to receive or agree to something, the other means to exclude or not include.
'Advice' is the guidance you receive (noun); 'advise' is the act of giving that guidance (verb).
'Affect' is the verb meaning to influence something; 'effect' is the noun meaning result or consequence.
Bryan López
Founder of SpeakUP Academy