The word as has several uses. It can be used as a conjunction. It can also be used as a preposition or an adverb.

As a conjunction, as shows time or cause.

As he was not ready, we went without him.

As I was about to step out, the telephone rang.

When as is used as a preposition, it is followed by a noun.

She works as a typist.

As can also be used as an adverb. In this case, it is followed by an adjective, an adverb or a quantifier like much or many.

I earn as much as he does.

Structures

As…as

The structure as…as… is used to say that two people or things are equal in some way.

She is as tall as her brother.
It was as good as I expected.
I can run as fast as you.

Note that in this structure the word following the first ‘as’ is usually an adjective or an adverb.

Just as

The expression just as can mean ‘equally’.

You need regular exercise, but a healthy diet rich in fruits and vegetables is just as important. (= … a healthy diet is equally important.)

The same as

Note this structure.

His opinion was the same as mine. (= We both had the same opinion.)

The expressions ‘as I was saying’, ‘as you know’ are common in spoken English. They are used to suggest that something is already known to the speaker or has already been talked about.

As you know, the farming sector is in deep trouble.
As I was saying, we expect to find newer opportunities this year.

As usual

The trains were late, as usual.