Contextual difference between That is why vs Which is why? Thus we say: You never know, which is why but You never know That is why And goes on to explain: There is a subtle but important difference between the use of that and which in a sentence, and it has to do primarily with relevance Grammarians often use the terms "restrictive" and "non-restrictive" when it comes to relative clauses
grammaticality - Is it incorrect to say, Why cannot. . . . ? - English . . . Since we can say "Why can we grow taller?", "Why cannot we grow taller?" is a logical and properly written negative We don't say "Why we can grow taller?" so the construct should not be "Why we cannot grow taller?" The reason is that auxiliaries should come before the subject to make an interrogative
Do you need the “why” in “Thats the reason why”? [duplicate] Good explanation of why it's optional in this case, although I'm not convinced that reason is the only reasonable antecedent of why For example, the explanation why is a common usage, and I don't think you can freely substitute that in that case either