Learn Spanish (Latin America): Tú no comes pan.

You would change tú to the formal "you", which is usted. Because the verb must match the pronoun, comes changes to come. So the formal version is Usted no come pan.

Why does Tú have an accent mark over the 'u'?

The accent mark shows that it means the pronoun "you" (tú). Without the accent mark, tu means "your" (as in tu pan, which means "your bread").

The English translation says "do not eat", but the Spanish just says no comes. Where is the "do"?

Spanish doesn't use "helper" words like "do" or "does" to make a sentence negative. To say "do not eat," you simply put no directly in front of the verb: no comes.

Why is it just pan and not el pan?

When you are talking about eating or drinking an unspecified amount of something (like "bread" in general, rather than a specific loaf), you don't need to use words like el (the) or un (a/some).