I've read the answers below, but there are still instances where I'm confused. For example, the test answers say that "Lo están llamando" is the correct translation of "They're calling him." However, I think I saw "They're writing him" translated as "Le están escribiendo." Both take the preposition "a" when the person being called or written is named, and both can use the preposition "to" in English. How is it possible to know that llamar takes a direct object, while escribir takes an indirect object?
Llamar (a alguien) is a transitive verb - we consider that who you call is a direct object, therefore it needs "lo/la".
Escribir is a transitive verb too when we are talking about writing someting (escribir una carta = to write a letter = escribirla, direct object la, referring to the letter), but is we use it as "to write to someone", that someone is an indirect object pronoun "le", "les".
The fact that you may have "to" or "a" adhered to the verbs doesn't always mean that the object is an indirect object - it can be a direct object too. Here are other verbs using a with direct object pronouns:
invitar a alguien (to invite someone)
Envidiar a alguien (to envy someone)
I hope this clarified it.
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