The meanings are very similar and sometimes you can use both with very little difference in meaning. "so far" is used when there is a progression which will definitely continue into the future. e.g. "Have you finished the book? Not quite. So far, I've read two thirds of it." "until now" is used when you want to indicate that the progression till now is changing in the present. e.g. "Until now, he has never been interested in learning other languages. But I think his new girlfriend from Fiji has sparked his interest!"

Hi Conchi, The difference between these two phrases is very, very subtle. You could use either in any context and be understood, but "until now" perhaps suggest that you are about to reveal what you remember right now, and "so far" may suggest that you may do so in the future. Examples: "I haven't studied much so far..." suggests: "I haven't studied much yet, and I may or may not begin studying soon." "I haven't studied much until now." suggests, "I wasn't studying, but now I intend to/now I am." I hope this helps!

Thanks so much for your helpful answers!