Learn German: Der Bus fährt jetzt ab. - The bus is departing now.

The verb here is abfahren (to depart), which is a separable prefix verb. In a normal sentence, the main part of the verb (fährt) goes in the second position, and the prefix (ab) gets kicked all the way to the end of the sentence.

Why does fährt have an umlaut (two dots) over the a?

The base verb is fahren (to drive or travel). It is a stem-changing verb, meaning the vowel changes from a to ä for the he, she, and it forms. Since der Bus is a singular noun acting as "he" or "it" here, fahren becomes fährt.

The translation says "is departing." Why isn't there a word for "is" in the German sentence?

German does not have a separate "is doing" (present continuous) tense like English does. The single verb fährt covers both "departs" and "is departing." Adding a word for "is" (like ist) would be grammatically incorrect here.

Can I start the sentence with jetzt (now) to emphasize the time?

Yes, you can! However, the conjugated verb (fährt) must always stay in the second position. If you put jetzt first, the bus and the verb must swap places to keep the verb second: Jetzt fährt der Bus ab.