Learn French: Beaucoup de citoyens défendront leurs droits contre cette loi. - A lot of citizens will defend their rights against this law.

The infinitive of the verb is défendre (to defend), which is a regular -re verb. To form the future tense, you drop the final -e to get the stem (défendr-) and then add the future endings. Because the subject here is Beaucoup de citoyens (equivalent to ils or "they"), we add the ending -ont to make défendront.

Why is it Beaucoup de citoyens and not Beaucoup des citoyens?

After expressions of quantity like beaucoup (a lot), un peu (a little), or trop (too much or too many), you always use de (or d'), never des. Even though citoyens is plural, the rule for quantities keeps it as just de.

Why does leurs have an "s" at the end here?

Leur means "their", and it must agree with the noun it describes. Because the citizens are defending multiple rights (droits), you add an s to make it leurs droits. If they were defending just one right, it would be leur droit.

Why do we use cette in cette loi?