Many years of teaching non-Farsi speakers has revealed to me that "Subjunctive" remains probably the most difficult tense to use by Persian learners.Following is some tips on how to use it:
(this is for those with at least an elementary level of Persian language).
1. When we want to say that an action will happen, but with doubt(we are not sure)
Example: shaayad(maybe) emruz Ali be khaaneye maa biayad.
2.When you wish an action to happen
Example: Kaash(I wish) emruz Ali be khaaneye maa biayad.
3.After some certain verbs or phrases implying "necessety", "intention","Ability":
Examples for No.3:
-maa baayad beravim.(Necessity:the word baayad(=must) expressing necessity, and the verb used after it is in subjunctive)
-man mikhaham beravam.(Intention: the word mikhaham(=I want) expressing intention, and the verb used after it is in subjunctive)
-Ali nemitavaanad beravad.(Ability: the word nemitavaanad(=he can`t) expressing ability, and the verb used after it is in subjunctive)
These are the most frequent use of subjunctive tense.Hope you find it helpful. should you have any questions regarding subjunctive or any thing else with your Persian do not hesitate to ask here, alternatively you may always write me at my email: firstname.lastname@example.org
To your improving Persian!