It has been observed that many people confuse the word ‘past’ for ‘passed’ even among the very educated people. Which explains why we need to pay close attention to our grammar. Sometimes, probably because we are not fully concentrating, we mistakenly use the inappropriate word in a particular context, thereby making the sentence lose its meaning. These words have a way of playing on us if we are very conversant with them.

The examples given below will give you an understanding of what context to use each of the words so that your message is well understood.
Past

The Oxford Advanced Learner’s Dictionary defines past as something gone by in time, something just ended or belonging to an earlier time.
Furthermore, it could serve as adjective, noun, preposition or adverb depending on the context in which it is used.

Past as an adjective
For example
- The days for late-coming to school are now past
- All the past senior prefects in my high school were male

Past as a noun- the time that has gone by or things that happened in an earlier time
For example
- Mary enjoyed relaxing in the park in the past
- Bill looked back at his past without any form of regret
- We have to go with the trend and not live in the past anymore.

Past as a preposition- referring to later than something as in telling the time.
For example
- Mr John left exactly half past seven
- The President read his speech at ten minutes past eight.

Another form of preposition- referring to ‘past’ as above or further than a particular point or stage or to the other side of somebody or something.
For example
- Abel walked past his friends without stopping to say hello
- The President is long past retirement age
- The teenage boy is long past his childish tantrums
- Carmen’s school is just past the state library

Past as adverb- referring to moving from one side of something to another
For example
- Sandra called out to Jason as he ran past the school field
- The ball ran past the goal post

Passed- A verb in the past tense
Passed is the past participle of ‘pass’. It can be both a transitive verb (in which a subject and object(s) is required) or an intransitive verb (in which no object is required).

‘To pass’ means ‘to move in the direction mentioned’, ‘to change from one state to another’, to move past or to the other side of someone or something’, ‘to depart’, or ‘to proceed’ ‘to achieve the required result in a test or an examination’, ‘to come to an end’.
For example
- My parents believe I will pass my final exams excellently
- The airport road was so narrow that the cars could barely pass
- After Joshua’s grandma passed on, he inherited her ranch
- The bridal procession passed slowly along the aisle to match the rhythm of the song

How do people confuse ‘past’ and ‘passed’
Some writers wrongly use ‘pass’ when they actually meant ‘past’
For example
- The pupils past the Proprietor’s office while marching to their class
The correct usage here is
- The pupils passed the Proprietor’s office while marching to their class

An easy way to know which to use is to write the sentence in the present tense assuming you are describing an event currently happening.
- The pupils pass the Proprietors office while marching to their class this morning or
- The pupils are passing the Proprietor’s office on their way to their class

This helps to determine which is more appropriate given the context you are using.

Credit: Daily Writing Tips