Adverbials
Adverbials include one-word adverbs (e.g. now, there, away, slowly), prepositional phrases (on Tuesday, at home, to the farm, etc); clauses (before you arrive, if he comes, etc); noun phrases (last week, last year, etc), and other types. They can classified into adverbials of time, place, manner, degree, etc. They usually 'modify' a verb, but some adverbials can play other modifying roles. Altogether adverbials are a very mixed group.

Manner Adverbs
A very large number of adverbs, including most adverbs of manner, are formed by adding -ly to and adjective. This is not possible if the adjective already ends in -ly, to form an adverb, for example:
Fast
Wrong: She ran fastly because the train was coming.
Correct: She ran fast because the train was coming.

Time Adverbs
After wrongly used
Wrong: The doctor is busy at present. Come again after.
Correct: The doctor is busy at present. Come again later.
'After' should not be used by itself as an adverb of time.

As of now
Wrong: As at now Musa is trying to build a house.
Correct: AS of now Musa is trying to build a house.

Before wrongly used
Wrong: I last visited the zoo before three years
Wrong: I last visited the zoo three years before.
Correct: I last visited the zoo three years ago.

When used as a preposition of time, 'before' means 'earlier than . . .':
There were powerful states in Africa before the coming of the Europeans.

'Before' can also be used as an adverb, usually coming at the end of a clause or a sentence, and meaning 'earlier than this':
I have never set eyes on him before.

But 'ago' is the right word meaning 'at such-and-such a point in time before the present'.