Double object (Le ... a mi hermana...?)
Do I say "le doy a mi hermana" or "doy a mi hermana"? When do I need the "le"? when do I double? is it necessary? Let's see what the official rules are so that you're never lost on this.
Then, only after you do some practice and see some examples, you might start asking yourself why do you sometimes see "le/les" (and sometimes "lo/la/los/las") when the person or thing (the direct or indirect object) is still in the sentence. Wasn't the purpose of the pronoun to replace it?
Well, there are some situations where we have to or like to double up, be extra clear, let's see what those are and how optional they are according to the RAE (Real Academia Española):
In these cases the official rules say you must have the pronoun (me,te,le,nos,os,les) in the sentence:
A mí, a ti, a él,… - emphasis
When we have these short references, we need the pronoun too, both direct and indirect objects.
The a mí, a ti,.. etc is not giving a lot of information, it's actually optional and we don't usually use it, but when we do it's to emphasise a contrast.
Me ve a mí – He/she sees me. You could just say me ve, the a mí part is the one we could get rid of. We probably have it to emphasise, something like "he/she sees me, not you".
Here me it's a direct object (if you don't understand this, check out the links I left above).
Nuria no te da el dinero a ti, me lo da a mí- Nuria doesn't give you the money, it gives it to me.
Both te and me are indirect in this case, and the lo is direct and replacing "the money" in the second sentence.
¿Te ha dado un regalo? ¡A mí no me da nada! - he/she has given you a gift? He/she doesn't give me anything!
Again, te and me are indirect and the a mí is to emphasise the fact that that person is treating us differently.
Noun pushed to the beginning - emphasis
If I have a phrase expressing the direct or indirect object at the beginning of the sentence, before the verb, then I also need to double up.
A tu hermano lo he visto ayer, pero a ti no te veo nunca - your brother, I've seen him yesterday, but you, I never see you.
In this example we double up because "a tu hermano" is at the beginning in the first sentence, and in the second part of the sentence, we double up too because we're using a ti, like we've seen in the previous section. Again, all of this repeating has the function of emphasis, here the comparison between how often I see each of the siblings.
Insisto, la tarta la llevo yo - I insist, the cake, I bring it myself.
Another direct object that I repeat because I'm insisting, with emphasis, that I want to bring the cake.
A mi padre siempre le he dicho la verdad - To my dad I've always told the truth.
I'm implying that to others I've lied, but never to my dad (indirect object).
Me gusta and similar verbs
Verbs like me gusta, me encanta, me fascina, me falta, etcetera are always a bit weird, and if you need a bit of context, you can learn about them a bit here.
These need the indirect object (me,te,le,nos,os,les) always, regardless of the presence of the person that has the opinion/is affected.
This means that the first time you mention the person you also have the pronoun, and after that you can just have the pronoun because we know who the person is already.
¿A tu hermana le gustan los dulces? or ¿le gustan a tu hermana los dulces? - Does your sister like sweets? (The second order is a bit more anusual)
If I ask another question regarding your sister I don't need to mention her again, I just use the le: ¿Y le gustan las tapas? - and does she like tapas?
These verbs include any sort of "influence" or "affection" verbs like molestar (to bother), divertir (to amuse), interesar (to interest), aburrir (to bore), parecer (to seem)...
Preferred (for emphasis)
Indirect object in general.
In the previous cases (pronoun or noun pushed at the beginning for emphasis) we could have an indirect object and in the case of the verbs like me gusta it's always an indirect object. But what happens with the rest of sentences with an indirect object in general?
It will be correct grammatically both ways, it's optional, although I'd say is preferred and more common to double up, sometimes to emphasise the indirect object, sometimes to clarify or avoid confusion with another noun.
(Le) han denegado la beca a Juan - They have denied the grant to Juan.
Julia no (les) da importancia a los problemas - Julia doesn't give importance to problems.
Todo, todos, toda, todas.
“Lo sé todo” (I know it all) “lo hago todo mal” (I do it all wrong), “las conozco a todas (I know them all), etc
Uno/a – "one" as impersonal.
Less high in the level of popularity, pretty much your choice.
"Si (lo) tratan mal a uno, no debe permitirlo" (if they treat one badly, one musn't allow it.
Don't double up- The rest of direct objects
If it's a double object and it's none of the mentioned cases ("todo/s", the article, "uno", "noun at the beginning"... for emphasis), we don't need the pronoun nor want it, it sounds off and it's wrong.
Ex: Los padres llevaron a los niños al parque.
We wouldn't say "Los padres los llevaron a los niños al parque".
(The parents took the kids to the park)
or with Comí tres croquetas I wouldn't say "Las comí tres croquetas"
(That means "I ate three croquetas", if you don't know what they are look them up and eat them at the first chance, they're a delicious tapa!!)
DO NOT confuse with:
Article + number
With the articles los and las + number you can see it both way. Really, the pronoun here is replacing a noun that is not here anymore, not doubling up in the same way as the other cases.
(Los) invité a los cuatro (I invited the four of them) Maybe it refers to "los cuatro amigos"
(Las) advertí a las seis (I warned the six of them).
Maybe it refers to "las seis alumnas" (that's why we have a feminine plural pronoun)
(Las) vi a las cuatro juntas (I saw the four of them together)
y.. eso es todo!
I would recommend to find examples of these in whatever you're reading at the moment (in Spanish, of course) to understand it even more in context, and then it's all about practice and awareness!
And again, I would recommend to check out the other links to do a sweep of the whole theory and make sure you know everything you need about these,
¡Hasta la próxima!