Modal verbs in Persian are used when the speaker wishes to change the grammatical mood of the sentence, namely necessity, ability, possibility, etc.

Foreword on the Subjunctive

The subjunctive form in Persian is used when one wishes to express something that it necessary, possible, uncertain or dependent on another verb/condition depending on the sentence in question and what it contains. The subjective form is identical to the present tense, except it uses be- (بِـ) at the beginning of the verb instead of the usual mi- (می). It is imperative that you do not mix up the subjunctive form with the present form. 

For your reference, the following is a guide on how to form the subjunctive in Persian:

1. Obtain the present stem.
   rikhtan (ریختن) --> riz (ریز)

2. Conjugate using be- (بِـ) and the verb ending according to the pronoun in question. Here, we'll use the pronoun man (من).
   riz (ریز) --> berizam (بریزم).

Politeness and the Subjective

Another thing to note on the the subjunctive form is that it is used when making polite requests. When requesting something from someone, and you wish to be polite, you would usually say something like "could you please, if you are able to, etc." In Persian, this is the same thing, and there are four main ways to do it.

• mishe... (می‌شه) "Could you..."

  • Written: mishe in rā bāz konid? (می‌شه این را باز کنید) "Could you open this?"

  • Spoken: mishe ino bāz konid? (می‌شه اینو باز کنید)

• momkene... (ممکنه) "Is it possible..."

  • mishe komakam konid? (می‌شه کمکم کنید) "Can you help me?"

• loftan... (لطفاً) "Please..."

  • Written: loftan dar rā bebandid? (لطفاً در را ببنیدید) "Please close the door."

  • Spoken: loftan daro bebandid? (لطفاً درو ببنیدید)

• khāhesh mikonam... (خواهش می‌کنم) "Please..." (more formal.)

  • Khāhesh mikonam injā beshinid. (خواهش می‌کنم اینجا بشینید) "Please sit here."

Now that we've covered the subjunctive form, which you must conjugate your verbs when using modal verbs in your sentence, we can move on to the four main modal verbs.

Bāyad (باید)

Bāyad (باید) means "must/should," it expresses an obligation or necessity. It is derived from the verb bāyestan (بایستن), which is considered archaic and almost never used to this day. Bāyad (باید) is not conjugated, and only the verb is the one conjugated, naturally into the subjunctive form. To negate bāyad (باید), add a na- (نَـ) in front of it.

Examples of this include:

• Bāyad berim (باید بریم), "we must go."

• Nabāyad benevisam. (نباید بنویسم), "I must not write."

Tavānestan (توانستن)

Tavānestan (توانستن) means "can/be able to," it expresses an ability to something. In spoken Persian, tavānestan (توانستن) is tunestan (تونستن). The past stem of tavānestan (توانستن) is tavānest (توانست), and the present stem is tavān (توان). In spoken Persian, this would be, respectively, tunest (تونست) and tun (تون). You would conjugate it as any you would any normal verb, and add a modal verb into the sentence if you please.

Examples of this include:

• Mitavānam/mitoonam in kāro bokonam. (.می‌توانم/می‌تونم این کارو بکنم) , "I can do this." (Note: In kāro (این کارو) is the spoken contraction of in kār rā (این کار را)).

• Nemitavāni/Nemitooni ino benevisi. (نمی‌توانی/نمی‌تونی اینو بنویسی), "You cannot write this." (Note: Ino (اینو) is the spoken contraction of in rā (این را)).

Khāstan (خواستن)

Khāstan (خواستن) means "want to," it expresses a desire to something. In written Persian, the present stem of khāstan (خواستن) is khāh (خواه), and the past stem is khāst (خواست). In spoken Persian, the present stem would just be khā (خوا). You would conjugate it as any you would any normal verb, and add a modal verb into the sentence if you please. (Note that in spoken Persian when you wish to refer to the present tense (e.g: you want something as of now,) khāstan (خواستن) is unusually conjugated in the past tense. Despite this not being grammatically correct, it is incredibly common. However, for our own examples, I will be using the grammatically correct option.))

Examples of this include:

• Mikhastam bastani bokhoram. (.میخواستم بستنی بخورم) , "I wanted to eat ice cream." 

• Nemikhāham/nemikhām ba ū harf bezanam . (نمیخواهم/نمیخوام با او حرف بزنم), "I don't want to talk to him." 

Momken ast (ممکن است)

Momken ast (ممکن است) means "may/might," implying a possibility, usually of the self, to do something or for something to happen. In spoken Persian, the ast (است) is turned into e (ه).

Examples of this include:

• Momken ast/e. (ممکن است/ممکنه) , "It is possible." 

• Momken ast bārān biāyid. (ممکن است باران بیاید), "It's possible [that] it will rain."