Use of "le"
Why is there is "le" in this sentence: Ella le está agregando azúcar a su té caliente. But there is no "le" in this sentence: La niña esta agregando novecientos mililitros de leche. What is the use of "le"?
As far as I understand when there is a direct and an indirect object in a sentence you will need to use the indirect object pronoun (even though it often seems superfluous).
Ella 'le' está agregando azucar a su té caliente. She (to it) is adding sugar to the hot tea.
Sugar is the direct object (that goes with the verb, ie. that's what's being added) and té caliente is the indirect object (it indirectly receives the action if the verb.
If your sentence only has a direct object, like your second example, there is no need for the indirect object or its associated pronoun.
Le is an indirect pronoun that must be used in Spanish. Go here for more info.
Ella le está agregando azúcar a su té caliente.
Le is a personal pronoun and used here as the indirect object, which indicates who is affected by the action. Without context, we don´t know who le represents. It could be: She is adding sugar to her (own) hot tea OR She is adding sugar to his/her/their hot tea.
La niña esta agregando novecientos mililitros de leche.
Here there is no mention of any person affected by the action, nor does it say to what the 900 ml of milk are added. This is a very elementary sentence and would only make sense as a short reply with context already known, ie: ¿Está la niña poniendo o quitando algo de la mezcla?
Here is a short explanation of the usage of le from the RAE. I hope you are able to understand it:
m. Gram. Empleo de la forma le y, con menos frecuencia les, de él en el acusativo masculino singular o plural cuando el pronombre representa a personas.
m. Gram. Incorrección consistente en emplear la forma le o les para el acusativo masculino singular o plural cuando el pronombre no se refiere a personas, o para el acusativo femenino singular o plural.
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