Learn Farsi: از امروز به بعد، باید گرایش اصلی خود را برای آزمون انتخاب کنید. - From today onwards, you must choose your main specialization for the exam.

It uses a circumposition, which is a multi-part preposition that surrounds a word. az means from, and be ba'd means to afterwards or onwards. You place the starting time right between them. You can use this pattern with other time expressions too, such as az farda be ba'd (from tomorrow onwards).

What exactly does gerayesh mean in this context?

While gerayesh literally translates to tendency or inclination, in an academic or professional context like taking an exam, it refers to a specialization, major, or concentration within a broader field of study.

Why is khod used here instead of a pronoun like shoma to mean your?

In formal Farsi, the reflexive pronoun khod (self) is frequently used instead of regular possessive pronouns when the possessor is the same as the subject of the sentence. Since the subject is you, which is indicated by the verb ending -id, khod acts as your own.

Why is the object marker ra placed right after khod?

The word ra marks the specific direct object of the sentence. Here, the entire noun phrase gerayesh-e asli-ye khod (your main specialization) is the direct object being acted upon by the verb entekhab konid (choose). The rule is that ra must always appear at the very end of the entire noun phrase it marks.