How to use the word 'As'

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.

for example

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

for example

 He was as hairy as a dog.

(He was like a dog he was so hairy.)

3. Preposition

To show the function of something or someone

He used the pen as a knife

(He used the pen like a knife)