An advert by the cigarette company Winston in the 1950s: This cigarette tastes good like a cigarette should. Correct: This cigarette tastes good as a cigarette should.

To show the relationship between words in a sentence. (Like is followed by a simple sentences without a verb)
You fight like a girl.
To connect ideas. (As is followed by a complex phrase that includes a verb)
You fight as if you were the strongest girl in the world.

More examples:



as + subject + verb

verb + like + noun / pronoun

It is as she has known me all my life.

Tom is like a young brother who has known me all my life.
She acts more as a sister would (act).

My friend acts more like a brother than a friend.
They are as smart as a fox.

They seem smart like a fox.
My sister thinks as I do.

My friend is like me.
Nobody dances as she does.

We speak like a native speaker.

Jane looks like a supermodel.

As means "in the manner" and is followed by a clause (subj + verb).

Like means "same form, appearance, kind, character" and is followed by a noun or noun phrase. The verb is often static, such as be, seems, looks, acts.