Learn Spanish (Spain): Las autoridades deben proteger a los ciudadanos. - The authorities must protect the citizens.

This is the personal a. In Spanish, when the direct object of a verb is a person or a group of people, you must put the preposition a right before it. Even though ciudadanos (citizens) is a large group, they are still people, so the rule applies.

What is the difference between using deben and tienen que in this context?

Both translate to "must" or "have to", but deber is often used for moral, ethical, or legal duties, which fits perfectly when describing the expected role of authorities. Tener que expresses a stronger, more absolute external necessity and is more common in everyday, informal speech.

Would the meaning change if the sentence said deben de proteger instead of just deben proteger?

Yes, it would change the meaning completely! Deber followed directly by an infinitive expresses obligation (they must protect). However, deber de followed by an infinitive expresses probability or an assumption. So, deben de proteger would mean "they must be protecting" or "they are probably protecting".

If I wanted to replace a los ciudadanos with a pronoun meaning "them", where would it go?

You can place the direct object pronoun los in one of two spots: either attached to the end of the infinitive (deben protegerlos) or right before the conjugated verb (los deben proteger). Both are perfectly natural and grammatically correct.