Molto, Tanto, Poco, Troppo

Molto, tanto, poco and troppo can be adjectives or adverbs.

• In front of a noun they are adjectives and agree with it, like buono: molto / molta / molti / molte. Ho molte amiche. (I have many friends.) C'è troppa gente. (There are too many people.) Ho poca fame. (I'm not very hungry.)
• In front of an adjective or after a verb they are adverbs and never change: Le mie amiche sono molto simpatiche. (My friends are very nice.) Lavori troppo. (You work too much.)

Quick test: if the word means "much / many" and stands before a thing, make it agree (watch out for feminine nouns like fame, gente, pazienza). If it means "very" or "a lot" and describes a quality or an action, keep -o.