We can use "like" and "as" to express similarity, but they are used differently: "like" is a preposition used before a noun or pronoun, while "as" is a conjunction used before sentences or prepositional phrases. We also use "as" but not "like" to express functions, jobs, or instruments - for example, we would say "he worked as a policeman" or "don't use my cup as an ashtray".

2.  We can use both “like” and “as” with the
meaning of similarity, but in a different way:
LIKE 
preposition. We
use it before a
noun or a
pronoun
•He looks like your
brother.
•They are dancing like us.

3. AS  conjunction.
Before sentences
or preposition
phrases
•Nobody knows him as I do
(sentence)
•On Monday, as on Friday,
the meeting will be at 6
o'clock (prepositional phrase)

4.  We use “as” and not “like” to express the
functions that people and things have, the
job they do or the instrument we use to do
something:
◦ Carl worked as a policeman for five years.
◦ Please don't use my cup as an ashtray.