In Persian, adjectives can be used as adverbs without any change. This is also typical of some other languages including German and Dutch. Adverbs modify a verb, an adjective or another adverb. They can even modify a whole sentence. In contrast, adjectives only modify a noun. The word بلند can mean both “loud” (adjective) and “loudly” (adverb) depending on the context. If it modifies a noun then it functions as an adjective, otherwise it functions as an adverb.

بلند آواز می‌خواند
Boland âvâz mikhwânad
He sings loudly
بلند modifies a verb. So it is an adverb
بسیار بلند آواز می‌خواند
Besyâr boland âvâz mikhwânad
He sings very loudly
بسیار modifies the adverb بلند. So it is an adverb
صدای بلند
Sedâ-ye boland
Loud voice
بلند modifies a noun. So it is an adjective
صدای بسیار بلند
Sedâ-ye besyâr boland
Very loud voice
بسیار modifies the adjective بلند. So it is an adverb

More examples:

خوب بخوابی
Khub bekhwâbi
Sleep well!
خوب خوابیدی؟
Khub khwâbidi?
Did you sleep well?
زود بر گرد
zud bar gard
Come back soon!

Comparative Adverbs

The relation between adverbs and adjectives also applies to comparative adjectives. For example, بهتر can mean “better” (adjective) or “well” (adverb) and آرامتر can mean “slower” (adjective) or “more slowly” (adverb) depending on whether it modifies a noun or another part of speech.

قطار کندتر از هواپیما حرکت می‌کند
Ghatâr kondtar az havâpeymâ harkat mikonad
Trains go more slowly than planes
آنها بردند چون بهتر بازی می‌کردند
Ânhâ bordand chon behtar bâzi mikardand
They won because they played better
می‌شود کمی آرامتر حرف بزنید؟
Mishavad kam-i ârâmtar harf bezanid?
Could you talk a bit more quietly please?

Place of Adverbs

Adverbs that modify an adjective come before it. This is always the case for attributive adjectives and typically for predicate adjectives as well. However, since Persian has free word order, an adverb can come anywhere in the sentence when it modifies a predicate adjective or the whole sentence.

دوست بسیار خوب
dust-e besyâr khub
very good friend
هوا کمی سرد است
Havâ kam-i sard ast
The weather is a little cold
کمی هوا سرد است
Kam-i havâ sard ast
The weather is cold a little
هوا سرد است کمی
Havâ sard ast Kam-i
The weather is cold a little