Lo vs Le

Question

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?

Accepted Answer

Hola Pete
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) 
Invité a María a mi fiesta -> La invité a mi fiesta
Envidiar a alguien (to envy someone)
Envidio a Pedro -> Lo envidio
I hope this clarified it.
Saludos
Inma

Answer

Hola Alice
You can only use the pronoun for "her", "la invité" because "a la fiesta" is not really a direct or indirect object (that "a" might make you think it is). 
Here in this other example you can:
 
Yo compré un regalo para mí.
Me lo compré.
Saludos
Inma

Answer

Hola! In the example

Invité a María a mi fiesta -> La invité a mi fiesta

Are there two direct objects (Maria, and la fiesta)? If so, can you substitute both with direct object pronouns? "La quiero invitar a la" doesn't sound right...

Thanks!