This word has many uses including the following.
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.
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 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.