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Dear teachers,

How do you know when a word like "before", "since", "like", "as", ect., is a preposition, adverb or conjunction? Would it be possible to give me some examples?

Many thanks,
Hela

'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.