There are two classes of infinitives in Persian: verbs ending in –dan, as in دیدن didan ‘to see,’ خوردن xordan ‘to eat’; and verbs ending in –tan, as in گفتن goftan ‘to say,’ نشستن nešastan ‘to sit.’

In the case of verbs ending in -dan, the infinitival suffix may follow:

• a vowel: دادن dādan ‘to give,’ شنیدن šanidan ‘to hear,’ گشودن gošudan ‘to open’
• the flap (/r/): خوردن xordan ‘to eat,’ مردن mordan ‘to die,’ کردن kardan ‘to do’
• the nasal /n/: کندن kandan ‘to cut off, dig,’ خواندن xāndan ‘to read’

The causative invariably has an [-ān-] ending before its infinitival suffix [-dan]: نشاندن nešāndan ‘to seat, set,’ خوراندن xorāndan ‘feed,’ خواباندن xābāndan ‘to put to sleep,’ سوزاندن suzāndan ‘to burn.’

In the case of verbs ending in –tan, the infinitival suffix follows a consonant: گفتن goftan ‘to say,’ نشستن nešastan ‘to sit,’ گذشتن gozaštan ‘to pass (by),’ سوختن suxtan ‘to burn.’

The infinitive in Persian is normally equated with the English infinitive, but it is more like the English gerund, that is, a verb in -ing when it does not represent a progressive form as in “Reading is good”: خواندن کار خوبی است xāndan kār-e xubi ast “Reading is good.” Therefore, in Persian an infinitive also functions as a gerund.

In Persian the only verb with an initial ī (ای) is the verb ایستادن intransitive verb ‘to stand (up); stop.’

The only verb in Persian with a single sound for the present stem is the verb آمدن āmadan ‘to come,’ with the present stem آ -ā-; as in, mi ā-yam ‘I come’ (with an euphionic [-y-] intervening between the stem and the personal suffix -am).