Question

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Answer 1

Let's translate the sentence first. It'll make it easier to explain:

"Mi hermana va a cantar la canción."
"My sister is going to sing the song."

Here, the VERB is "going to sing."

And what's she going to sing?
The song.

The SONG is the DIRECT OBJECT.

So, what we need to do....is take the word "canción,"
and replace it with a DIRECT OBJECT PRONOUN.

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In Spanish, there are four direct object pronouns:

LO
LOS
LA
LAS

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We need to use LA,
because "canción" is SINGULAR, and FEMININE.

And all we need to do is stick the DIRECT OBJECT PRONOUN
onto the end of the VERB.

The VERB, in the Spanish sentence, is CANTAR.

So, here's what we'll say:

"Mi hermana va a cantarla."

You see?
We just put LA onto the end of CANTAR.
It's that easy.

It means, "My sister is going to sing it."