Learn Spanish (Latin America): En el restaurante, yo como postre y tú comes arroz. - At the restaurant, I eat dessert and you eat rice.

We use them here for contrast. Because the sentence compares what two different people are doing, connecting them with y (and), adding yo and tú emphasizes the difference: I eat dessert, and you eat rice.

Do I need a comma before y (and) like I often do in English?

No, in Spanish you generally do not use a comma before y when it connects two related thoughts, even if they each have their own subject and verb like yo como and tú comes.

Why does the verb change from como to comes?

In Spanish, the ending of the verb changes to match who is doing the action. Como ends in -o, which goes with yo (I). Comes ends in -es, which goes with the informal tú (you). Both come from the regular verb comer (to eat).

Why does tú have an accent mark over the u?

The accent mark tells apart the subject pronoun tú (you) from the possessive word tu (your). Without the accent, it would look like you were saying 'your eat' instead of 'you eat'.