To choose the correct form, ask yourself: Is it followed by a verb?
If the answer is yes, use as.
As I mentioned previously, the meeting will begin at 9 am.
It sounded as though she would be late.
In each of these examples, the word as is followed by a subject and verb (I mentioned; she would be). In this context, as is a kind of connecting word called a conjunction, and conjunctions can be followed by verbs.
Like, on the other hand, is not a conjunction. So, in the above examples, as is the only correct choice, right? According to most language guides, it is. However, don’t be surprised if you hear a native speaker, in informal speech or writing, use like followed by a verb – for example, like I said before… or it sounded like she would be late.
For your purposes, as an English language learner, the safest choice is to use as, not like, before a subject and verb.
Now, back to our original question: Is it followed by a verb?
If the answer is no, like and as are both possible, but with a difference in meaning.
If there is no verb, just a noun or noun phrase, you want to use a preposition rather than a conjunction. Use like for the meaning similar to, and use as for the meaning same as.
Compare these examples:
You look like your sister. (You look similar to your sister.)
As your sister, I’d like to give you some advice. (This might be said by a woman speaking to her brother; she is his sister, so as, not like, is correct.)
You sound like a teacher! (This might be said to a student who is explaining a grammar point to another student. It only makes sense if the person referred to is not really a teacher.)
It’s up to her, as the teacher, to design the curriculum. (She is the teacher.)
To recap, ask yourself if it’s a conjunction (followed by a subject and verb) or preposition (no verb; only a noun or noun phrase). If it’s a conjunction, use as. If it’s a preposition, ask yourself if the meaning is similar to or same as. If it’s similar to, use like. If it’s same as, use as.
Now you can use like and as with confidence, like a native speaker!