Learn Swedish: Klassen börjar klockan nio. - The class starts at nine o'clock.

In Swedish, you don't need a preposition to say "at [time] o'clock" for scheduled events like this. You just say klockan nio (literally "the clock nine"). If you wanted to say "at around nine," you would then use a preposition: vid nio.

If I want to emphasize the time and start with "At nine o'clock", how does the sentence change?

If you move the time to the beginning of the sentence, Swedish requires the verb to stay in the second position (this is known as the V2 rule). The sentence would become: Klockan nio börjar klassen. Notice how the verb börjar comes right before the subject klassen.

Could I use the verb startar instead of börjar here?

While people would understand you, börjar is the most natural verb for events, school classes, and movies beginning. The verb startar is usually reserved for machines, engines, or launching a new project.

Why does klassen end in -en?

Klassen means "the class". Because the basic noun is an en-word (en klass), you attach -en to the end to make it definite. This is the standard Swedish way of saying "the" for this gender of nouns.