Only #1 and #4 are correct.
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teechrOnly #1 and #4 are correct.But I got one passage written by a foreigner, on which it said "Why blogging has become so popular today". Is this the wrong form?
teechrWhy blogging has become so popular today is a mystery to me. correct!Why blogging has become so popular today? incorrect!Thank you soooo much!
AlpheccaStars #2 and #3 are usable as noun clauses.Do you know why blogging has become so popular today?He wrote a good article on why blogging is so popular today.Thank you so much!