Digo "A Maria LA atropello un carro" pero por que digo "LE compré un libro a Maria" "??? DIOSSSSSS😂

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No estoy seguro pero creo que es porque el "LA" se refiere a María y el "LE" es un artículo que complementa la acción del pasado...
Es un poco complicado de entender pero es cuestión de acostumbrarse a como se usa eso es todo. Espero haberte ayudado

LA se refiere a María mientras que LE se refiere a ti o la persona que compró un libro a María, en este caso sería algo similar a "yo compré un libro a María" otro ejemplo en el que no siempre LE se refiere a yo es....
"LE diré a mamá (yo diré a mamá)
que LE has mentido (que tú mentiste a ella)"
si digo que LE he mentido sería (yo mentí a ella).

Because of what I told you in another question you did: the direct objects are "lo/la" and the indirect objects are "le".

In the first sentence (Un carro atropelló a María-A María la atropelló un carro), "un carro" is the subject, and there has to be a direct object, so "a María" becomes the direct object, as there can never be an indirect object without a direct object. So if you sustitute "a María", the sentence would be "La atropelló un carro" and you can repeat the complements like you write in your example (A María la atropelló un carro).

In the second sentence (Compré un libro a María), the subject is "yo" (although it is omited), the direct object is "el libro" and the indirect object is "a María". So, when you sustitute "a María" the sentence is "Le compré un libro", and you can repeat complements, so it's correct "Le compré un libro a María".

In summary, what you have to see is if there are direct objects and indirect objects, because the direct objects will turn into "lo/la" and the indirect objects into "le". You also have to know that the indirect object doesn't exist unless you have direct object.

If you see "a 'somebody'" (for example "a María"), this doesn't mean that it's an indirect object, it can be direct or indirect. And if there is another complement, like "el libro", this would be the direct object and then "a María" would become the indirect object.

Some examples:

-Yo compro el libro a María
I see that "el libro" is direct object and "a María" is indirect object, so I know that "el libro" can be sustituted with "lo/la" (lo because is masculine) and "a María" with "le".
->I sustitute "el libro": Lo compré a María
->I sustitute "a Maria": Le compré el libro
->I sustitute both: Se lo compré a María ("Le" turns into "se" because it sounds better)
->I sustitute both and I repeat complements: Se lo compré el libro a María

-Un carro atropelló a María
I see that there is only one complement, so "a María" is direct object as there can't be indirect object without direct object, and as it is direct, it can be sustitute with "lo/la" (la in this case because is feminine).
->I sustitute "a María": Un carro la atropelló
->I sustitute "a María" and I repeat complement: Un carro la atropelló a María