Indirect Object Pronouns

[AngelicaDeAlquezar]

I don't know why this would be an exercise, but I can only say that this is hard, even for native speakers. The "personal a" makes it very confusing for most of us.
We are taught to distinguish direct object from indirect object by asking "¿qué?" and "¿a quién?" correspondingly, which works for some sentences like:

- (Le) di un chocolate a mi mamá. (I gave a chocolate to my mom.)
DO question: ¿Qué (le) di a mi mamá? (What did I give to my mother??
Answer: Un chocolate. (A chocolate)
IO question: ¿A quién (le) di un chocolate? (To whom did I give a chocolate?)
Answer: A mi mamá. (To my mom.)

But then, there is the "personal a", which we use to separate a transitive verb from a person. This is a way to show respect for every person, and it's extended to animals that we care for. So the question "¿a quién?" becomes confusing for DO and IO:

- Ellos invitan a Susana.

The DO question "¿qué?" is not working here, because the verb doesn't "pass" its action on something. Yet, if we ask "¿a quién?" there is a valid answer, but the interpretation that because of this we have an indirect object is wrong.
We have to examine the verb and whether it's transitive or not.
"Ellos invitan" without a complement, makes no sense, so "invitar" is a transitive verb, and the person invited is the direct object.

- Llevo al perro al veterinario. (I take the dog to the vet.)
Again, asking "¿a quién?" has a logical answer, but "el perro" is the recipient of the action of the verb, so the answer "al perro" is not an IO, but a case of "personal a".

- El caníbal se comió al explorador. (The cannibal ate the explorer.)
Even when it's such a disrespectful action, like eating someone, we still use "personal a" to separate the person from the verb, but this is still a transitive verb, and the explorer is the DO.

@Oldman: Keep asking questions. It's going to become clear some time.

[Rusty]

Yeah, the link that Oldman provided in post #8 contained very good information as I read it through. Very good.
Much the same question that Oldman said he found on a quiz was one of the topics of that discussion. And it became the main topic after a while.
Oldman's quiz has something wrong with it if it asked him to substitute an indirect object pronoun for the direct object Susana. Perhaps that was just a mistake.

(As I said in post #10, when the verb invitar is used with a different meaning (treat, buy, pay for), Susana is the indirect object., but that meaning was not being used in the quiz.)

Sorry to have added confusion to confusion.