Learn Farsi: شیرینی نمی‌خورم تا وزن کم کنم. - I do not eat sweets so that I lose weight.

The conjunction tâ (تا), meaning "so that" or "in order to," introduces a goal or purpose. In Farsi, expressing an intended outcome requires the subjunctive mood rather than the indicative (factual) mood. That's why we use the present subjunctive konam instead of the normal present tense mikonam.

I learned that present subjunctive verbs start with the prefix be-. Why is it missing from kam konam (کم کنم)?

The verb used here is the compound verb kam kardan (to decrease/lose). In modern Farsi, the subjunctive prefix be- is almost always dropped from the "doing" part of compound verbs. So, instead of saying kam bekonam, you simply say kam konam.

Could I use barâ-ye inke (برای اینکه) instead of tâ (تا) in this sentence?

Yes, absolutely! Both tâ and barâ-ye inke can mean "so that" or "in order to," and both require the following verb to be in the subjunctive. You could say širini nemi-xoram barâ-ye inke vazn kam konam. tâ is just a shorter option that is very popular in everyday speech.

The English translation says "sweets" (plural), but isn't širini (شیرینی) singular?

Yes, širini is grammatically singular. However, when talking about a general category of things in Farsi—like eating sweets, buying apples, or drinking tea—the singular noun form is usually used. You don't need to add the plural suffix unless you are talking about specific, distinct pieces of pastry pieces of sweets.