Learn Farsi: برای اینکه این اصطلاح را درک کنید، باید واژگان آن را یاد بگیرید. - In order to understand this idiom, you must learn its vocabulary.

It follows the phrase برای اینکه (barāye in-ke), which means "in order to" or "so that." Conjunctions that express purpose or intention trigger the subjunctive mood in the following verb, which is why it doesn't have the standard می (mi-) prefix.

Why does the word واژگان (vājegān) have a "g" sound in it when the singular word is واژه (vājeh)?

واژگان is the plural of واژه (word). In Farsi, when you add the animate or formal plural suffix ـان (-ān) to a word ending in a silent ه (like واژه), that silent ه drops and a گ (g) is inserted to bridge the gap between the vowels. Here, it translates collectively to "vocabulary."

The second clause uses باید (bāyad), so the verb should be subjunctive. Where is the بـ (be-) prefix on یاد بگیرید (yād begirid)?

It is there, but because یاد گرفتن (yād gereftan) is a compound verb, the prefix doesn't go at the very beginning. In compound verbs, the subjunctive بـ (be-) prefix attaches only to the verbal part of the compound. So, گرفتن becomes بگیرید (begirid), giving us یاد بگیرید.

Why do we see the object marker را (rā) twice in this sentence?

Because the sentence is made up of two distinct clauses, and each has its own specific direct object. In the first part, the specific thing being understood is (this idiom). In the second part, the specific thing being learned is (its vocabulary). Since both objects are definite, both require (rā).