This word has many uses including the following.
In comparisons
As is used in making comparisons. It is used to say that people or things are equal or similar in some way.
- I am as tall as you.
- I can run as fast as him.
- This box is twice as heavy as that one.
Note that after as, we usually use object pronouns (e.g. him, her, them). Subject pronouns are used in a formal style and they are usually followed by verbs.
- I am as tall as she is.
- I can run as fast as he can.
As can mean when or while.
- I saw him as I was going out. (= I saw him when I was going out.)
- Just as I opened my eyes, I saw a strange sight. (= Just when I opened my eyes, I saw a strange sight.)
- I saw him as he was falling.
Because
As can also mean because.
- As he was not there, I left a message with his mother. (= Because he was not there, I left a message with his mother.)
- As you won’t come with me, I will go alone.
As can mean in the same way.
- Do as I do.
- While in Rome, do as Romans do.
As much
As much can mean so.
- I thought as much. (= I thought so.)
As long as = provided that
- I will come as long as I don’t have to do any work.
Note that in clauses introduced by as long as, we use a present tense to refer to the past.