Some words are sometimes used as adjectives and sometimes as adverbs. Examples are: loud, fast, high, next, back, little, hard, ill, best, quick, early, only, enough, much etc.

We heard a loud noise. (Loud – adjective)
He talked loud. (Loud – adverb)

He is a fast bowler. (Fast – adjective)
A fast (adjective) bowler bowls fast (adverb).

She lives next door. (Next – adjective)
When I next see him, I will speak to him. (Next – adverb)

Adverbs with two forms

Some adverbs have two forms. While one of these forms looks exactly like the adjective, the other form ends in –ly.

Examples are given below.

He talked very loud.
He talked very loudly.

Here the adverbs loud and loudly modify the verb talked.

Note that in some cases the two forms of the adverb may have different meanings.

Compare:

John worked hard. (Here the adverb hard means diligently.)
I could hardly see anything. (Here the adverb hardly means scarcely.)

He arrived late. (Opposed to early)
I have not seen him lately. (Here lately means recently.)

I am pretty impressed with her performance. (= I am quite impressed with her performance.)
She is prettily dressed. (= She is neatly/elegantly dressed.)

Some adverbs are used as nouns after prepositions.

They live far from here. (Here the adverb here is used as a noun meaning ‘this place’.)
He comes from there. (Here the adverb there is used as a noun meaning ‘that place’.)
By then the thief had escaped. (Here the adverb then is used as a noun meaning ‘that time’.)
Since when have you taken to drinking? (When=what time)

Adverbs are sometimes used like adjectives modifying nouns. This, for example, happens when some participle or adjective is understood.

The then Prime Minister = The Prime Minister then ruling
A down train = A down-going train
The above statement = The statement given above

The as an adverb

In the following sentences the is not the definite article. It is actually an old demonstrative pronoun used as an adverb.

The more the merrier
The fewer the better
The earlier the better
The sooner the better

Note that the can be used as an adverb only with an adjective or another adverb in the comparative degree.