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.
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
The combination sí que is very common and is similar to the English phrase "does indeed”:
La cintura de Shakira, sí que tiene movimiento
Shakira's waist does indeed have movement
Caption 53, Alberto Barros - Cargamento Colombiano
The following is an interesting example. A literal translation of a tí sí te contamos is "you, we do tell," but a more accurate translation in English uses the future tense:
A ti sí te contamos, pero a nadie más.
You we will tell, but nobody else.
Caption 20, Guillermina y Candelario - El mundo de los juguetes perdidos - Part 1
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. ¡Sí, loco, otro más! Si estás para eso, ¿no?
I have to ask you a favor. Yes, dude, another one! That's [indeed] what you are for, right?
Caption 27, Muñeca Brava - 30 Revelaciones - Part 4
¿Puedo? -Claro, si te dije que son pa' los dos.
May I? -Of course, I [indeed] told you that they are for both of us.
Caption 58, Muñeca Brava - 8 Trampas - Part 9
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. ¿Yo te di mi teléfono, no?
Oh, whenever you want, no, I'd love to. I gave you my phone [number], right?
Caption 27, Muñeca Brava - 2 Venganza - Part 3
The word si as an emphatic affirmation is also commonly used to express a protest or to contradict someone. In this case it's equivalent to the English word "but”:
Si tú estás igualita. -Si yo estoy más fresca que una lechuga.
But you are exactly the same. -But I'm fresher than a [head of] lettuce.
Caption 8-9, NPS No puede ser - 1 - El concurso - Part 6
Si yo lo estoy diciendo hace rato ya, hombre.
But I've been saying that for a while already, man.
Caption 71, 75 minutos - Del campo a la mesa - Part 4
No, no, no, si yo ya me voy. No, no, quédese, quédese. -¿Seguro?
No, no, no, but I'm already leaving. No, no, stay, stay. -[Are you] sure?
Caption 4, Muñeca Brava - 3 Nueva Casa - Part 10