The infinitive is the basic form of a verb. For example: to do, to go. In Persian, infinitives end in -tan or -dan. Originally, all infinitives ended in -tan but the ending has evolved into -dan after vowels and the consonants n and r:

Original Ending
داشتن
dâshtan
to have
بستن
bastan
to close
رفتن
raftan
to go
گفتن
goftan
to say
Ending After Vowels
آمدن
âmadan
to come
دادن
dâdan
to give
دیدن
didan
to see
شدن
shodan
to become
بودن
budan
to be
Ending After n and r
خواند
khwândan
to read; to sing
کندن
kandan
to dig
بردن
bordan
to take; to win
خوردن
khordan
to eat

Apocopated infinitive

Infinitives have a short form, which is technically called apocopated infinitive. It is formed by removing -an from the full form and is used, for example, in the formation of simple future tense:

Apocopated infinitive Full infinitive
خواهم رفت
khwâham raft
I will go
خواهم رفتن
khwâham raftan
خواهی رفت
khwâhi raft
You will go
خواهی رفتن
khwâhi raftan
خواهد رفت
khwâhad raft
He/She/It will go
خواهد رفتن
khwâhad raftan

Negative infinitives

The negative form of an infinitive is made by prefixing na-:

Positive Negative
بودن
budan
to be
نبودن
nabudan
not to be
رفتن
raftan
to go
نرفتن
naraftan
not to go
آمدن
âmadan
to come
نیامدن
nayâmadan
not to come

Passive infinitives

The passive voice is formed from past active participle and the auxiliary verb شدن (shodan). Passive infinitives are mainly formed from transitive verbs.

Active infinitive Passive infinitive
خواندن
khwândan
to read
خوانده شدن
khwânde shodan
to be read
خوردن
khordan
to eat
خورده شدن
khorde shodan
to be eaten

Regarding compound verbs made with کردن (kardan), their passive infinitive is formed by replacing کردن (kardan) with شدن (shodan).

Infinitive Past passive participle
باز کردن
bâz kardan
to open
باز شدن
bâz shodan
to be opened, to become open
پاک کردن
pâk kardan
to clean
پاک شدن
pâk shodan
to be cleaned, to become clean

Infinitive as a Noun

In Persian, infinitives can act as a noun and accept various prefixes and suffixes. For example, when an infinitive accepts suffix -i, it transforms into an adjective which means "worthy of", "-able", …

Infinitive Derivation
دیدن
didan
to see
دیدنی
didani
worth seeing
دوست داشتن
dust dâshtan
to love
دوست‌داشتنی
dustdâshtani
loveable
باور کردن
bâvar kardan
to believe
باورکردنی
bâvarkardani
believable
باور نکردن
bâvar nakardan
not to believe
باورنکردنی
bâvarnakardani
unbelievable

Infinitives can also function as what it is known in English as gerund or -ing form.