A lot, but if you can remember the following, it will be a lot easier to pick which one you need to use.
The word past can be used as an adjective, preposition, noun or adverb and is generally used to locate something in time and/or space.
The word, passed is the past participle of the verb “to pass”. It can be an intransitive verb (one which doesn’t require an object) or a transitive verb (one which requires both a subject and one or more objects). “To pass” means “To proceed, move forward, depart; to cause to do this.”
If you’re like me, it’s easier to remember the rules if you can see them in action so I’ve given some examples below.
Past as an adjective.
1. “just gone or elapsed”.
Example: Long past are the days of silent movies.
2. “having existed or taken place in a period before the present: Bygone.
Example: “In the past, dinosaurs roamed the earth.”
3, “A past life, history or course of action.”
Example: His past is an open book for all to read.
4. “having served as a specified officer in an organization.”
Example: All past CEO’s of the company were women.
Past as a preposition.
Definition: beyond the age for or of; at the farther side of; in a course of direction going close to and then beyond.
Don’t you think you’re past the age to play with dolls?
The old church is just past the hardware store on the right.
I drove past the house where I grew up.
It is half past noon.
Past as a noun:
Definitions: time gone by; something that happened or was done in the past; the past tense of a language; a past life, history or course of action, especially one that is kept secret.
In the past, we only paid five cents for a loaf of bread.
It does no good to live in the past.
In my past life, I was a dog.
Past as an adverb
Definition: so as to reach and go beyond a point near at hand
The dog sped past the cat.
Passed (primarily used as the past tense of ‘to pass’)
The hours passed quickly. (Intransitive: subject “the weeks” and no object).
I passed all of my tests! (Transitive: subject “I” and object “my tests”.)
When do “past” and “passed” get confused?
Often, writers muddle the words past and passed in sentences such as:
The teens passed through a town on their way to the beach.
It’s common to see this written as:
The teens past through a town on their way to the beach. However, the word should be passed, as (in this sentence) it’s the past participle of the verb “to pass”.
An easy way to tell is to rewrite the sentence in the present tense, as though you’re describing something which is happening currently:
The teens pass through a town on their way to the beach. or
The teens are passing through a town on their way to the beach.
However, if you wrote:
‘The teens drove past a town on their way to the beach’, it would be correct to use the word past as the verb in this sentence is “drove”, and the “past” is acting as an adverb.
Unusual uses of the word “passed”
These are called phrasal verbs and are easy to remember:
To pass out of existence; to die
Example: The old man passed away.
To be accepted as or believed to be:
Example: She looked so young she passed for a teenager.
To pass off:
Example: The counterfeit money passed for original.
to lose consciousness:
Example: The athlete passed out on the field.
To let go by; reject
Example: He passed up a chance for a promotion.
To leave out; disregard
Example: He passed over the peas.
Okay, so there you have it. I hope this cleared up some confusion. If not, feel free to pop over any questions you might have.