Did you know that the Spanish words sí (usually meaning "yes") and si (usually meaning "if") also have special uses that are for emphatic purposes? Let's look at some examples.
 

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The word sí (yes) is used in a similar way to the repetition of the word "do" to express that someone indeed did something. For example, when someone says "you did not do it," one can reply, "I did do it." Well, in Spanish, you use the word sí (the orthographic accent is important here) in a similar way: a declaration such as tú no lo hiciste (you did not do it) can be answered with yo sí lo hice (I did do it).
 
Like the repetition of the word "do" in English, this use of sí has a purely emphatic effect. You could easily answer tú no lo hiciste (you did not do it) with a simple yo lo hice (I did it), but using yo sí lo hice (I did do it) is way more common. Let's look at some examples so you can learn how to throw in that emphatic sí in conversation:

Ah claro, ahora sí lo entiendo hija, ¡qué torpe soy!

Oh, of course, now I do understand it, girl. How clumsy I am!

Caption 57, 75 minutos - Del campo a la mesa - Part 7

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The combination sí que is very common and is similar to the English phrase "does indeed”:

Traigo mi cargamento -La cintura de Shakira

I bring my shipment -Shakira's waist

sí que tiene movimiento -De pura salsa

does indeed have movement -Of pure salsa

Captions 48-49, Alberto Barros - Cargamento Colombiano

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On the other hand, the word si (without the orthographic accent), commonly used in conditional clauses, can also be used to indicate that you are affirming something very emphatically. It's not always easy (or necessary) to translate it into English, but in the following examples we added "indeed”:

Te tengo que pedir un favor.

I have to ask you a favor.

¡Sí, loco, otro más! Si estás para eso, ¿no?

Yes, dude, another one! That's [indeed] what you are for, right?

Captions 42-43, Muñeca Brava - 30 Revelaciones

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In the following example, a more literal translation of si me encanta could be "indeed, I love it" or just "I do love it," but we used "I'd love to" (me encantaría in Spanish), which better suits the context:

Ah, cuando quieras, no, si me encanta.

Oh, whenever you want, no, I'd love to.

¿Yo te di mi teléfono, no?

I gave you my phone [number], right?

Caption 62, Muñeca Brava - 2 Venganza

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The word si as an emphatic affirmation is also commonly used to express a protest or to contradict someone. It could be translated as "but,” but it doesn't actually need a translation, as you can see in the following examples:

¿Cuál es tu burla? ¡Si tú estás igualita!

What are you making fun of? [But] You are the same!

¡Si yo estoy más fresca que una lechuga!

[But] I'm fresher than a [head of] lettuce.

Captions 8-9, NPS No puede ser - 1 - El concurso

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Si yo lo estoy diciendo hace rato ya, hombre.

But I've been saying it for a while already, man.

Caption 71, 75 minutos - Del campo a la mesa - Part 4

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