Learn Farsi: آنها تند می‌دوند چون عجله دارند. - They are running fast because they are in a hurry.

It just connects the two parts without changing the word order! چون (chon) simply sits between the two independent thoughts. The second part, عجله دارند, still follows the normal Farsi rule of putting the verb (دارند) at the very end of its clause.

Why do we use the verb دارند (they have) for "they are in a hurry"?

In Farsi, you don't say you "are" in a hurry; instead, you "have" haste or a hurry. That's why we use عجله (hurry) with the verb داشتن (to have). Since the subject is "they", it conjugates to دارند.

English says "running fast", so why is تند (fast) placed before the verb here?

In Farsi, adverbs that describe how an action is done usually go right before the verb. So instead of "they run fast", the structure is literally "they fast run" (تند می‌دوند).

The English translation says "are running". Why doesn't the Farsi sentence use a "to be" verb here?

The standard Farsi present tense covers both "they run" and "they are running". The prefix میـ attached to the verb دوند shows that the action is happening right now or happens regularly. You don't need an extra "to be" verb like you do in English to show that an action is ongoing.