Learn Farsi: به جای اینکه وقت هدر دهیم، کارهای سودمند انجام می‌دهیم. - Instead of wasting time, we do useful tasks.

The phrase be jâ-ye inke (instead of) acts as a trigger for the present subjunctive. Because you are talking about an action you are not doing (wasting time), it is treated as hypothetical. So, you must use the subjunctive hadar dahim rather than the indicative hadar midahim.

If hadar dahim is subjunctive, why does it not start with the usual be- prefix?

Hadar dâdan (to waste) is a compound verb made of a noun and the light verb dâdan (to give). In many compound verbs, especially those using dâdan or kardan, Farsi speakers often drop the subjunctive be- prefix. You could technically say hadar bedahim, but hadar dahim sounds much more natural.

Why is the second verb anjâm midahim in the regular present tense when the first verb is subjunctive?

The first part of the sentence is governed by be jâ-ye inke, which requires the subjunctive because it describes an alternative action that is not actually happening. However, the second part (anjâm midahim) is a statement of fact about what you are actually doing, so it stays in the normal present indicative tense.

What does the -mand part of sudmand mean?

The suffix -mand means possessing or having and is used to turn nouns into adjectives. Here, means profit or benefit, so literally means possessing benefit, or useful. You have probably seen this before in words like (elderly) or (wealthy).