'Before/since' as adverbs:
I have never seen him BEFORE.
Ever SINCE, she has been afraid of spiders.
- as prepositions:
BEFORE the War, he was a doctor.
He's lived here SINCE 1940.
- as conjunctions:
I arrived just BEFORE he left.
I've known about the problem SINCE it first started.
Of the above three word-classes, 'like' functions formally only as a preposition (although it functions informally as a conjunction). 'As' is essentially a conjunction, although in certain cases it has prepositional affinities.