Learn German: Die Probe fängt um neun Uhr im Theater an. - The rehearsal starts at nine o'clock in the theater.

In German, time expressions (um neun Uhr) almost always come before place expressions (im Theater). You might hear teachers call this the "Time, Manner, Place" rule (or TeKaMoLo). English is more flexible, but German is strict about time coming first!

Why is the word an sitting all by itself at the end of the sentence?

The verb for "to start" here is anfangen, which is a separable verb. In a standard main clause, the prefix (an) splits off from the main verb (fängt) and gets pushed to the very end of the sentence, after all the time and place details.

Why is it im Theater and not in das Theater?

The word im is a contraction of in + dem. Because the rehearsal is happening at a set location (answering "where?"), the preposition in triggers the dative case. The neuter noun das Theater changes to dem Theater, and in dem is almost always shortened to im.

Could I start the sentence with the time instead?

Yes! You can emphasize the time by putting it first: Um neun Uhr fängt die Probe im Theater an. Notice that the verb (fängt) must stay in the second position, so the subject (die Probe) has to move to the third spot, right after the verb.