An adverb describes a verb. It provides information about how the verb is carried out. Adverbs usually come before the verb. They do not decline.

bene – well
celeriter – quickly
falso – falsely
ibidem – at/in the same place
ideo – therefore
imperpetuum – forever, in perpetuity
item – likewise
libere – freely
non – not
nunc – now
prius – before
quiete – quit
semper – always
sepe – often
sicut – just as
tandem – at length
ubi – where, when
videlicet – namely, to wit, that is

Notice that many English adverbs end in ‘-ly’.