As you probably know, Persian verbs have past and present stems, e.g. خورد and خور from خوردن.
To form the present subjunctive, the prefix "بِ" (usually optional) is added to the present stem plus an ending that reflects person and number, e.g. بِخورَم.
Now, بودن in present subjunctive form (e.g. باشم) acts as an auxilary verb in forming the past subjunctive of other verbs, when combined with their past participle (e.g. داشته from داشتن), e.g. داشته باشی.
Note that there is no equivalent verb for this in English. For example, ".نباید مشکلی داشته باشی" translates to "You shouldn't have a problem."
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