Unit 106. As (time) – “I watched her as she worked.” As (reason) – “As I was feeling tired, I went to bed.”
As (time): two things happening together
You can use as when two things happen at the same time or over the same period of time: I watched her as she opened the letter.
As they walked along the street, they looked in the store windows.
Turn off the light as you go out, please.
We use as especially for two short actions happening at the same time: George arrived as I left. (= he arrived and I left at the same time)
We all waved goodbye to Tom as he drove away in his car.
You can also use just as (= exactly at that moment): George arrived just as I left.
Just as I sat down, the phone rang.
We also use as when two changes happen over the same period of time: As the day wore on, the weather got worse.
I began to enjoy the job more as I got used to it.
As (time): one thing happening during another
You can say that you did something as you were doing something else (= in the middle of doing something else).
When we use as in this way, both actions are usually quite short:
The man slipped as he was getting off the train.
Jill burned herself as she was taking the cake out of the oven.
The thief was seen as he was climbing over the wall.
You can also use just as: Just as we were going out, it started to rain.
I had to leave just as the conversation was getting interesting. For the past continuous (was getting / were going, etc.) see Unit 12.
Note that we use as only if two actions happen together. Do not use as if one action follows another:
When I got home, I took a bath, (not as I got home)
As sometimes means “because”:
As I was feeling tired, I went to bed early. (= because I was feeling tired)
As they live near us, we see them quite often.
As tomorrow is a national holiday, all the stores will be closed.
As we had nothing better to do, we watched television the whole evening.
For as and like see Unit 107. For as… as see Unit 99.