As

As is a conjunction. We use it before a clause, and before an expression beginning with a preposition.

• He did as he was told.
• He fought as a tiger does.
• Nobody knows her as I do.
• In 1939, as in 1914, everybody seemed to want war.

In informal English like is often used as a conjunction instead of as. This is very common in American English. It is not generally considered correct in a formal style.

• Nobody knows her like I do.
  

As in the sense of though

• Though he was young, he fought valiantly.
• Young as he was, he fought valiantly.
• Though the elephant was strong, it could not face the swift leopard.
• Strong as the elephant was, it could not face the swift leopard.

So as

This structure is used to talk about purpose.

• We went early so as to get a good seat.
• He climbed up a tree so as to get a good view of the procession.

Such as

We use such as with a noun to introduce examples.

• My doctor told me to avoid fatty foods such as bacon or hamburgers.
• I have bought many fireworks such as rockets.