The Difference Between As v Like

First let's look at the rules, and then some examples so we can understand when to use As and Like in context.

LIKE

The rule for "Like" is when something is similar to another thing. Not the same but similar.

Examples

a. "You are kind and sweet... just like your mother" – You are not your mother but your are similar to her.

b. "My friend plays the guitar like a professional" – So he's not a professional but he plays the guitar in a similar way to a professional. He plays very, very well!

c. "She speaks English like a native speaker" – She is not a native speaker but her level of English is so high that she is similar to a native speaker.

AS

The rule for "As" is when we're describing the job or function of something.

Examples

a. "I work as an English Teacher" – That's my job, that's my function.

b. "He plays the guitar as a professional" - That is his job, he is a professional guitarrist. 

c. "I use one of the bedroom in my house as an office" – The function of this bedroom is being an office.

CHUNKS!

I highly recommend you learn some uses of As and Like in chunks. Chunks are group of words that go together all the time, and if you learn in chunks you will speak English with much more fluency. If we learn a few common chunks, we don't analyze the grammar too much, but instead we just speak naturally... like native English speakers do! So here are common chunks using As and Like.

As I said

E.g.  As I said, It's good to learn in chunks. 

As if

E.g. He was very scared. He acted as if he had seen a ghost.

As...As

E.g. My brother is not as tall as me. I am not as old as my brother. 

Like this /Like that  

Which means "in this way".

Look Like/ Sound Like / Smell Like

E.g. You look like your Mom / When you speak you sound like your Dad / It smells like garlic in here... did you eat garlic yesterday?