There are three common ways to use the word 'as'
1. Conjunction- To join two clauses
a. To join two clauses when two things are happening at the same time.
I was singing as I was waiting for the train.
b . To state the reason for an action ( it can be used like 'because or since')
I was late as I slept in.
2. Comparative adverb
He was as hairy as a dog.
(He was like a dog he was so hairy.)
To show the function of something or someone
He used the pen as a knife
(He used the pen like a knife)