Learn Farsi: او عجله دارد چون امروز خواب ماند. - He is in a hurry because he overslept today.

The word چون means "because" and it simply joins two complete sentences together without changing their word order. You start with the main sentence او عجله دارد (he is in a hurry), add چون, and then attach the reason امروز خواب ماند (today he overslept).

Why does the sentence use دارد for being in a hurry?

In Farsi, you do not say that you "are" in a hurry, you say that you "have" a hurry. The word عجله means hurry or rush, and دارد is the he/she form of the verb to have. So, او عجله دارد literally means he has hurry.

How exactly does خواب ماند mean overslept?

This is a compound verb made of two parts. خواب means sleep and ماند is the past tense of the verb to stay or to remain. Literally, it means he remained asleep, which is the Farsi way of saying he overslept.

Why is there no word for "he" after چون?

Because Farsi verb endings already tell you who is doing the action, subject pronouns like او can be dropped. We already know we are talking about him from the first half of the sentence, so it is perfectly natural to leave it out before امروز خواب ماند.