Learn Croatian: Svatko mora poštovati zakon. - Everyone must respect the law.

Just like in English where we say "everyone is" (not "are"), the Croatian pronoun svatko is treated as grammatically singular. Therefore, it takes the third-person singular verb form mora.

Why doesn't the word zakon change its ending here, even though it's the direct object?

It is actually in the accusative case because it receives the action of poštovati (to respect). However, zakon (law) is a masculine inanimate noun (it's not a living being). For masculine inanimate nouns, the accusative form is identical to the nominative dictionary form.

The verb poštovati is in the infinitive here. How would you say "I respect" in the present tense?

Verbs that end in -ovati generally drop that ending and add -ujem in the present tense. So "I respect" becomes poštujem, and "he/she respects" is poštuje.

Can I use svi instead of svatko to mean "everyone"?

Yes, but the grammar changes slightly! Svatko means "everyone" as a singular concept, while svi is the plural form meaning "all (people)". If you used svi, the verb would have to be plural: Svi moraju poštovati zakon.