Ola! "Si" is the equivalent for "if" in english. Example : Si "A" alors "B" / If "A" then "B". It's about conditions. "Tellement" I like to think that "tellement" means "so much". "Tant" "Tant" and "tellement" are very similar words. It's all about the "shade" you wanna add to it. It would means something like "to that extent". Overall, a mistake that I often see is : "I miss you so much" = Tu me manques 'si tant'. Which is wrong! In that case, we say : Tu me manques tant OR tellement. The reason why you cannot add "Si" is that both "tant" and "tellement" implies that it's already "so much". A little bit like in english : It is very tasty. It is delicious. But you cannot say : It is very delicious because delicious already suppose the "very" part in it. Hope I helped :) Alex

Tu trouveras la règle ici : http://niveau-avance.blogspot.fr/2012/10/si-et-tellement.html