truly, madly, deeply

Adverbs go with verbs* and adjectives go with nouns (or pronouns).

a true friend   - adjective describing the friend

I love you truly - adverb saying how I love

You can often add -ly to an adjective to make it an adverb

mad madly

deep deeply

true truly  

Note the final -e drops off. 

Yours truly,

Same with due duly. 

Duly noted!

Or double a final l

brutal brutally

Or add -ally for ease of pronunciation

tragic tragically

Not all adverbs end in -ly

good (adjective) well (adverb)

Frequently you hear an adjective used where it should be an adverb.

They are now able to clear the runway twice as quick. (Heard on BBC news)

It should of course be "quickly" since it is the verb clear that is done quickly.

Scottish football managers are frequently heard saying, "the boys done good!"  Aaaagh!

Consider the following

He was born premature.

He was born prematurely.

Which is correct?

The first uses the adjective premature going with the pronoun "he". It is much better to say "he was premature"

The second example is correct.

* Adverbs can also go with adjectives and adverbs

she is very pretty
that worked quite well

More on adverbs another time.