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.
Long object at 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)...
The rest of the cases are optional, you can see them with just the indirect object/direct object the first time, and then just the pronoun replacing it, or both of them together.
(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.
However, amonst this big mayority of cases when it's not essential, there are some where it's preferred to double up still, and you can choose to ignore those or go with the favourite choice:
Preferred but not mandatory
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.
"Si lo tratan mal a uno, no debe permitirlo" (if they treat one badly, one musn't allow it), "uno ya no sabe cuales son las reglas" (one doesn't know what are the rules anymore).
With the articles los and las + number.
(Los) invité a los cuatro (I invited the four of them), Las advertí a las seis (I warned the six of them).
And... 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!