Learn Farsi: شما نمی‌توانید این مشکل را تنها حل کنید.

In Farsi, when you use a modal verb like "can" or "to be able to" (tavânestan), the second verb must be in the present subjunctive form. Instead of saying "you cannot to solve", Farsi literally says "you cannot that you solve". Here, the modal nemi-tavânid (you cannot) requires the main action to take the subjunctive form hal konid.

If hal konid is in the subjunctive, why doesn't it have the be- prefix?

The verb "to solve" is a compound verb: hal kardan (literally "solution doing"). When making compound verbs subjunctive, Farsi speakers usually drop the be- prefix from the "doing" part. So, it becomes hal konid instead of hal bokonid, though both are correct.

Why are the two verbs, nemi-tavânid and hal konid, separated by the rest of the sentence?

In Farsi, the main verb almost always goes at the very end of the sentence. The modal verb (nemi-tavânid) comes early, right after the subject (shomâ), while the main action (hal konid) is pushed to the end. The object ("this problem") and adverbs ("alone") are sandwiched between them.

Why do we need the word râ right after in moshkel (this problem)?

The word râ is used to mark a specific direct object. Because you are pointing out " problem" () rather than talking about solving problems in general, you must add to show exactly what is receiving the action of the verb.