Learn Hardcore Latin: Domina servum vocat, et servus statim venit. - The mistress calls the servant, and the servant comes at once.

Because domina is in the nominative singular, which is the normal case for the subject of the verb. The verb vocat is 3rd person singular (she calls), so a singular subject like domina fits perfectly.

Why is servum spelled with -um? What does that ending do?

Servum is accusative singular of servus (slave). The accusative is commonly used for the direct object—the person or thing being acted upon. So domina servum vocat = the mistress calls the slave (the slave is receiving the action of calling).

If servum is the object in the first clause, why is it servus in the second clause?

Because the slave has a different grammatical role in the second clause.

In domina servum vocat, the slave is the object → accusative (servum).
In servus statim venit, the slave is the subject → nominative (servus).

Latin marks roles mainly by case endings, not by word order.