Learn Farsi: بهتر است عصبانی نشوی. - It is better that you do not get angry.

This is from the verb شدن (to become). Its present stem is شو (shav). To make it negative subjunctive, we replace the usual subjunctive prefix بـ (be-) with the negative prefix نـ (na-). Then we add the ending ی (-i) for the informal you. So it becomes نـ + شو + ی which makes نشوی (that you do not become).

Why do we use the subjunctive form here instead of the regular present tense?

The phrase بهتر است (it is better) expresses a recommendation or a wish. In Farsi, when you give a recommendation using phrases like it is better that, you must use the subjunctive for the verb that follows.

Why does the Farsi sentence use نشوی (you do not become) instead of a verb for being?

In Farsi, changing into an emotional state uses the verb شدن (to become). While English often says to be angry or to get angry, Farsi specifically uses عصبانی شدن (to become angry) to describe the action of getting mad.

Where is the word for you in this sentence?

The subject you is built directly into the verb! The ending ی (-i) on نشوی tells us the subject is the informal singular you. Adding the independent pronoun تو (you) is optional and usually left out in everyday speech.