Passed vs Past
The word past locates something in time, and sometimes in space. It can be
used as an adjective, noun, or adverb.
Past as an adjective
The first definition which the OED gives for past as an adjective is Gone by in time; elapsed; done with; over. For example:
When attributed to a group of people, past can also mean Having served ones term of office; former. (OED)
And in grammar, we have more examples of past being used as an adjective, such as in past tense and past participle.
Past as a noun
The main meaning for the noun form of past, given by the OED, is The time that has gone by; a time, or all of the time, before the present.
Past as a preposition
As a preposition, past can mean: Beyond in time; after; beyond the age for or time of; (in stating the time of day) so many minutes, or a quarter or half of an hour, after a particular hour. (OED)
It can also be used for location: Beyond in place; further on than; at or on the further side of; to a point beyond. (OED)
Past as an adverb
The first meaning the OED cites for past being used as an adverb is So as to pass or go by; by. For example:
Passed is the past participle of the verb to pass. It can be an intransitive verb (one which doesnt 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. (OED) This can refer to movement forwards in time, in space, or in life (such as to pass an examination).
For example:
Often, writers muddle the words past and passed in sentences such as:
Its common to see this written as:
But the word should be passed, as (in this sentence) its the past participle of the verb to pass. An easy way to tell is to rewrite the sentence in the present tense, as though youre describing something which is happening currently:
However, if you wrote:
Its correct to use past. The verb in this sentence is walked, and the past is acting as an adverb.
Most of the time, passed is a verb, as described above. There are a few occasions when it can be used as a noun or an adjective, though. For example: