Many English adverbs are formed by adding -ly to an adjective (e.g. most + ly = mostly;calm + ly = camly; etc.). Many Portuguese adverbs are formed in a similar manner by adding -mente to the singular feminine form of the adjective:

sincero (sincere) + mente = sinceramente (sincerely)
horrível (horrible) + mente = horrívelmente (horribly)

However, several common Portuguese adverbs have the same form as the masculine singular adjective:

breve (or brevemente) = soon or shortly
raro (or raramente) = rarely
barato = cheap or cheaply
caro = dear or dearly

When two or more adverbs ending in -mente are used together (strung together) in the same sentence, only the last word using the -mente ending is used as such. All others use the feminine form of the adjective. For example:

She talked slowly, clearly and pleasantly.
Ela falada vagarosa, clara e amavelmente.

but never...

Ela falada vagarosamente, claramente e amàvelmente.