When you want to describe how an action is done in English, you usually add the suffix -ly to an adjective, turning a word like slow into slowly. In Italian, the equivalent ending is -mente.
However, you cannot always just stick -mente onto the end of a dictionary word. English speakers often trip over this because Italian requires you to look at the feminine form of the adjective first, or tweak the ending if the adjective ends in certain letters. Words like lentamente and facilmente all follow a specific set of patterns to become adverbs.
For most adjectives, which end in -o in their default dictionary form, you must first change them to their feminine singular form (ending in -a) before adding -mente.
For example, to say slowly, you take lento, change it to lenta, and add the suffix to get lentamente. The same happens with ovvio becoming ovvia for ovviamente, and esatto becoming esattamente. If you want to do something perfectly, you use perfettamente.
L'uomo cammina molto lentamente.
(The man walks very slowly.)
Ovviamente, non posso venire alla festa.
(Obviously, I cannot come to the party.)
Questo vestito ti sta perfettamente.
(This dress fits you perfectly.)
Adjectives already ending in -e
Some adjectives already end in -e in their singular form. Because these adjectives have the exact same form for both masculine and feminine, you do not need to change the final vowel. You simply add -mente to the word.
A common example is veloce. You just attach the suffix directly to the end to create velocemente.
Dobbiamo finire il lavoro velocemente.
(We have to finish the work quickly.)
The drop-e rule for -le and -re
There is a catch to the previous rule. If an adjective ends in -le or -re, you usually have to drop the final e before attaching -mente. This makes the word flow better and avoids clunky pronunciation.
For instance, facile loses its final e to become facil, creating the adverb facilmente. Similarly, probabile becomes probabilmente, and particolare becomes particolarmente.
Abbiamo risolto il problema facilmente.
(We solved the problem easily.)
Probabilmente pioverà domani mattina.
(It will probably rain tomorrow morning.)
Questo libro è particolarmente interessante.
(This book is particularly interesting.)
Summary
To turn an Italian adjective into an adverb, remember these three steps. If it ends in -o, change it to -a and add -mente. If it ends in -e, just add -mente. But if it ends in -le or -re, drop the final e before adding the suffix.
Forming adverbs with the '-mente' suffix