You would ask me - what' so difficult in saying "that is why"? We know the word for "that/this/it," we need no "is" word, and we know how to say "why" - and off we go! Not really. Don't do it, don't translate literally! I've heard so many students do it - are you one of them? ;) We have a very special word - just one word! - that stands for it. And this magic word is - לכן (LAKHEN).*

Example:
אף פעם לא שמעתי את המילה הזאת קודם, לכן לא הבנתי אותך
(I've never heard this word before, that's why I didn't understand you)