What is the difference between 'that is why' and ' which is why eg ' You can never know what will happen on a journey ' commo here''that is why' or 'which is why' you should have a travel insurance?

Answer 1

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. A relative clause provides additional information about the noun it describes, but it may be considered relevant or irrelevant to the overall point of the sentence. In other words, a restrictive relative clause, which often begins with that, is usually considered essential or restrictive. Relative clauses beginning with which may contain non-essential information and would be considered non-restrictive.

Or simply: Use which when it introduces a new clause in the same sentence. Use that when it begins a new sentence. Thus we say "You never know, which is why..." and we say "You never know. That is why..."