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'Parlare forte', 'andare piano': adjectives that work as adverbs

In Italian you usually turn an adjective into an adverb by adding -mente: chiaro becomes chiaramente, veloce becomes velocemente. A small group of everyday adjectives can skip that step. They sit right after a verb and describe how the action is done: parla forte (speak loudly), corre veloce (runs fast), vai piano (go slowly), parliamo chiaro (let's speak plainly).

English speakers already know this trick from run fast or work hard, so the construction itself feels natural. The trouble comes from Italian agreement. You are used to making adjectives match the noun, so it is tempting to say parlano forti or parliamo chiari. When these words act as adverbs, they never change their ending.

Word list
altomA1 tall volare alto = to fly high forteA1 strong piove forte = it's raining hard veloceA1 fast correre veloce = to run fast chiaromA2 clear parlare chiaro = to speak plainly forteA2 loud parlare forte = to speak loudly leggeromA2 light mangiare leggero = to eat light pianoA2 slowly andare piano = to go slowly bassomB1 low volare basso = to fly low
How it works

Put the bare adjective, in its masculine singular form, straight after the verb. It describes the action, not the person, so it stays the same whoever is speaking, running or driving. Singular or plural, masculine or feminine, the word does not move.

Le ragazze parlano forte.
(The girls are speaking loudly.)

Rule of thumb: if you could replace the word with a -mente adverb, or with an English adverb such as loudly, fast or clearly, keep it invariable. This only works with a limited set of words, though. You cannot do it with any adjective you like. Stick to the common combinations below, plus a few fixed ones such as lavorare duro (to work hard).

The words

forte is the most frequent one. With verbs of speaking and sound it means loudly: parlare forte, gridare forte, ridere forte.

Maria, per favore parla più forte.
(Maria, please speak louder.)
I bambini gridano forte in giardino.
(The children are shouting loudly in the garden.)

With other verbs forte means hard or with strength. You will hear it a lot with rain, and with pressing or holding.

Oggi piove forte.
(It's raining hard today.)
Tienimi forte la mano.
(Hold my hand tight.)

veloce means fast, and it goes naturally with verbs of movement and with work done quickly.

Mio fratello guida troppo veloce.
(My brother drives too fast.)
Mangia veloce, il treno parte tra dieci minuti!
(Eat fast, the train leaves in ten minutes!)

piano is the opposite of both veloce and forte. It means slowly with movement and quietly with sound.

Vai piano, la strada è bagnata.
(Go slowly, the road is wet.)
Parlate piano, il bambino dorme.
(Speak quietly, the baby is sleeping.)

chiaro mostly appears with parlare and dire, meaning plainly or openly, with nothing hidden.

Parliamo chiaro: non abbiamo abbastanza tempo.
(Let's speak plainly: we don't have enough time.)
Te lo dico chiaro, non vengo.
(I'm telling you plainly, I'm not coming.)
Watch out

The typical mistake is to make the word agree with the subject. Le ragazze parlano forti and parliamo chiari are wrong. The same words do agree when they describe a noun, so compare:

Le mie sorelle sono forti, ma parlano piano.
(My sisters are strong, but they speak quietly.)

With veloce you will sometimes hear i ragazzi corrono veloci. That version describes the boys themselves, as in running and being fast. It is correct, but the invariable corrono veloce is always safe when you mean how they run.

Also remember that the -mente forms still exist. Parlare chiaramente and guidare velocemente are fine, just a bit more formal. Pianamente is not used for slowly at all, so always say piano.

Summary

A few common adjectives, above all forte, veloce, piano and chiaro, can follow a verb directly and work like adverbs, much like English run fast. When they describe how something is done, use the masculine singular form and never make it agree: le ragazze parlano forte, parliamo chiaro, andate piano.