Thus we say: You never know, which is why. but You never know. That is why.
Understanding the Context
And goes on to explain: There is a subtle but important difference between the use of that and which in a. As to why I do, I really don't know. (=.but if you want to ask why I do, I don't know.) I am going for sure. As to whether Jane will go along too, you will have to ask her yourself.
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Key Insights
English is fun, as. May 9, 2012 · It’s a headline, first of all, where some grammatical rules are different anyway. So this is not a sentence, but a noun phrase: (This section tells you) why to use page-level.