Say how things are done: lentamente, bien, mal
English adds -ly; Spanish adds -mente — but always to the feminine form of the adjective.
lento → lenta → lentamente (slowly)
tranquilo → tranquila → tranquilamente (calmly)
fácil → fácilmente (easily) — adjectives without a separate feminine just add -mente
If the adjective has a written accent, the adverb keeps it in the same place — -mente never steals the accent.
rápido → rápida → rápidamente (quickly)
fácil → fácilmente (easily)
Ella conduce rápidamente. (She drives quickly.)
Result: -mente adverbs are the rare Spanish words with two stressed syllables — RÁ-pida-MEN-te.
Bueno and malo do not take -mente. Their adverbs are bien (well) and mal (badly) — exactly like English good → well.
Mi hermana cocina muy bien. (My sister cooks very well.)
Hoy he dormido mal. (I slept badly today.)
Never say "canta bueno" — just as you would never say "she sings good" in careful English.
When two manner adverbs are joined by y, only the last one takes -mente. The first stays in its feminine adjective form.
Habla lenta y claramente. (He speaks slowly and clearly.)
Trabaja rápida y eficazmente. (She works quickly and efficiently.)
English repeats -ly on both words; Spanish trims the first one. It is elegant once you expect it.
Adjectives describe nouns and agree with them. Adverbs describe verbs and never change.
Ella es una conductora rápida. (She is a fast driver.) — adjective, agrees
Ella conduce rápidamente. (She drives quickly.) — adverb, invariable
If the word answers "how is it done?", you want the adverb.
Traps for English speakers
These are the errors English speakers make most often.