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,. I have flunked the exam, that is why i am attending coaching classes.
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I have flunked the exam, which is why i am attending coaching classes. Is there any change in the meaning of sentence. Why should the first person pronoun 'i' always be capitalized? I realize that at one time a lot of nouns in english were capitalized, but i can't understand the pattern of those.
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I don't know why, but it seems to me bob would sound a bit strange if he said, why is it that you have to get going? eliminating 'that' before 'bob' would seem to be more in context.