Learn Farsi: با او دعوا نکن، او خسته است.

In Farsi, many verbs are "compound verbs," made of a noun plus a helper verb like کردن (kardan - to do). "To fight" is دعوا کردن (da'va kardan). To make a negative command, you only change the helper verb. You take the present stem of کردن, which is کن (kon), and add the negative prefix نـ (na-). This gives you نکن (nakon).

Why doesn't the command نکن (nakon) have a personal ending?

When you are giving a command to a single person informally, the verb uses the bare present stem with no extra personal ending. Since the stem is کن (kon), adding the negative prefix نـ (na-) just makes it نکن (nakon). If you were speaking formally or to multiple people, you would add the plural ending ـید (id) to make it نکنید (nakonid).

How do we know او (u) means "he" here and not "she"?

We actually don't know just from the grammar! Farsi pronouns are completely gender-neutral. او (u) can mean "he" or "she" depending entirely on the context of who you are talking about. It is translated as "he" here just for the English example, but it could equally mean "she".

Is it necessary to repeat the word او (u) in the second half of the sentence?

No, it is optional. In Farsi, the subject is already built into the verb. The word است (ast) means "is" (for he/she/it). Because this makes it obvious who you are talking about, you could drop the second (u) and simply say: .