As can be used a subordinating conjunction. As a subordinating conjunction it is used in different ways.

As = when

As can be used as a subordinating conjunction of time. In this case, it means more or less the same as when.

As is used as an alternative to when when we are / were comparing two short actions that happen / happened at the same time.

In this case, we often use as with just.

Ann arrived just as I was about to leave. (= Ann arrived when I was about to leave.)

Just as I sat down to read I started feeling sleepy.

As = because

As can be used as a subordinating conjunction showing cause or reason. As is preferred when the reason is already known to the reader or listener.

As-clauses usually come at the beginning of the sentence.

As Alice was the eldest child, she had to take care of the family after the death of her father. (This is something you can expect.)

On the other hand, because puts greater emphasis on the reason. It may also introduce new information.

Alice has to work hard because her husband has lost his job.

Because-clauses do not usually come at the beginning, but it is possible, though.

As it was raining we didn’t go out.

Susie divorced her husband because he had been cheating on her.

As is also used to talk about two situations which develop or change together.

As I get older I get more tolerant.

As prices rise, the demand for higher wages becomes shriller.