The French word si can be an adverb or a conjunction. Either way, si has several meanings and is used in numerous French constructions.
Si = if
Si is the French word for "if":
Je ne sais pas si je veux y aller
I don't know if I want to go
Dis-moi si ça te conviendra
Tell me if that will work for you
Et si je ne suis pas fatigué ?
And if I'm not tired?
Si j'étais riche, j'achèterais une maison
If I were rich, I would buy a house
(lesson on si clauses)
Si = so
Si can be used as an intensifier:
Je suis si fatigué
I'm so tired
J'ai si faim
I'm so hungry
Je ne savais pas qu'il était si mignon
I didn't know he was so cute
Si = as, so
Si can make a comparison:
Il n'est pas si intelligent qu'il pense
He's not as smart as he thinks
Ce n'est pas si facile
It's not as easy as that, It's not that easy
Si = while, whereas
Si can put two clauses in opposition:
S'il est beau, sa femme est laide
Whereas he is handsome, his wife is ugly
Si tu es gentil, ton frère est méchant
You're kind, while your brother is mean
Si = however, no matter how
Si can be followed by a subjunctive clause to express a concession:
Si beau qu'il fasse, je ne peux pas sortir
No matter how nice the weather is, I can't go out
Si gentil que tu sois, je ne t'aime pas
However kind you are, I don't love you
Si = yes
Si means "yes" in response to a negative question or statement:
Tu ne vas pas venir ? Si (je vais venir)
You're not going to come? Yes (I am going to come)
N'as-tu pas d'argent ? Si, j'en ai
Don't you have any money? Yes, I do
Jeanne n'est pas prête. Si, si !
Jeanne isn't ready. Yes (she is / I am)!
Si = did I hear correctly, is this what you're asking?
If someone asks a question and you're not sure (or can't believe) you heard correctly, you can request confirmation or clarification by repeating what you did hear with the word si:
Si j'ai faim ?
(Are you asking) if I'm hungry?
(You couldn't really hear the question)
Si je veux quoi ?
You're asking if I want what?
(You're not sure you heard correctly; you heard "Do you want a free TV?")
Si j'ai combien d'enfants ?
You're asking if I have how many kids?
(You didn't hear "how many," or you heard "Do you have 7 kids?")
Et si = what if, how about
In informal French, et si is often tacked on the beginning of a suggestion (with the verb in the imperfect):
Et si on allait au ciné ?
How about going to the movies?
Et si tu amenais ton frère ?
Why don't you bring your brother?
Et si on parlait d'amour ?
What if we talked about love?