Learn Farsi: شیر باید استریل باشد، بنابراین قبل از اضافه کردن مایه، آن را گرم می‌کنیم. - The milk must be sterile; therefore, we heat it before adding the rennet.

banâbar-in means "therefore." It simply connects two independent clauses and does not force a special verb mood. That is why the following verb, گرم می‌کنیم (we heat), is in the regular present indicative tense, rather than the subjunctive.

Why do we use باشد instead of است for "is" in the first half?

The word باید (must) requires the verb that follows it to be in the subjunctive mood. باشد is the present subjunctive form of بودن (to be), so باید استریل باشد translates to "must be sterile."

How does Farsi translate the "-ing" form in "before adding"?

Farsi uses the full infinitive of the verb where English uses a gerund (-ing word) after prepositions. Here, اضافه کردن (to add) is used. It is linked to its object, مایه (rennet), using an invisible ezafe vowel, which makes it sound like "ezâfe kardan-e mâye".

What is the purpose of آن را right before the final verb?

آن means "it" and refers back to the milk (شیر). Since "it" is the specific direct object that we are heating, it must be followed by the specific direct object marker را.

Is مایه the exact technical word for "rennet"?