As is a subordinating conjunction. It is used to give the reason for something. It is similar to because and since.

• As she was not feeling well, she did not go to work.
• As they were not ready, we went without them.
• As it is raining, we cannot go out now.

In a less formal style, we can express the same ideas using so.

• She was not feeling well so she did not go to work.
• They were not ready so we went without them.
• It is raining, so we cannot go out now.

Note that as and because are not normally interchangeable.

As can also show time.

As is often used with simple tenses to talk about two situations which develop or change together.

• As we get older, we get wiser.
• As I get older, I get happier.

As can also be used to talk about two simultaneous short actions.

• As I opened my eyes, I saw a strange sight.