Si

Definition:


1. "so" 

Il fait si chaud! - It is so hot!

2."if" 

Si tu veux. - If you want.

In this sense, the i of si is replaced by an apostrophe if it is followed by a word that starts with a vowel.

S'il fait beau. - If the weather is OK.

3. "yes" 

If a question is in the negative, instead of saying oui - yes, the affirmative answer is si - yes (indeed).

Il n'y a pas de plage à Paris! - Si, je t'assure. 
There isn't a beach in Paris! - Yes, there is! I assure you.

Tu ne vas pas aller à la plage maintenant? - Si, j'y vais. 
You're not going to the beach now, are you? - Yes, I am! 

Tu ne veux pas de café. - Si, j'en veux. 
You don't want a coffee. - Yes, I do!