Hi !!
To form an adverb, the suffix <em>-mente is </em> added to the end of a feminine singular form of the adjective.
If the adjective ends in an -o, the -o is changed to an -a
<em>exemples =</em>
nuev<u>o</u> ==> nuev<u>a</u> => nuev<u>a</u><em>mente</em>
rápid<u>o</u> ==> rápid<u>a</u> => rápid<u>a</u><em>mente</em>
fácil => fácil<em>mente</em>
breve => breve<em>mente</em>
final ==> final<em>mente</em>
hoping this helped :)