Is it improper grammar to say 'how come' instead of 'why'?

Answer 1

"How come?" or "Why?"Popular Opinion- Technically it would not necessarily be wrong but many English teachers and professors prefer why because it is more mature-like and understandable without sounding like a child.

Answer- Although it is not improper, to use "how come" it in the place of "why" is an American idiom that some teachers would like to eliminate. It is most often used today in informal speech, even though it appears in educated journals in the 19th century. It is a shortened version of the phrase, "how does it come to be..."

"How come", in its proper usage, differs from "why" in that "how come" is factive, whereas "why" is not. "How come" presupposes its complement; "why" does not. Anastasia Marie Conroy of the Linguistics Department at the University of Maryland discusses these and other differences at length in her article The Semantics of How Come: A Look At How Factivity Does It All.http://www.ling.umd.edu/~staceyc/Conroy%20UMWPiL%2014.pdf

To further illustrate the above, the question, "How come I have to stay after school?" indicates that the speaker wants to know the reason that they must stay after school. On the other hand, the question "Why do I have to stay after school?" could mean the same thing or that the speaker would like to know why he or she should stay after school, as in "Why do I have to stay after school (when I can get my work done at home)?"

Other Answers-

Yes it is improper. Example: "How come I have to stay after school?" to "Why do I have to stay after school?" You can see the difference. The reason one must speak fairly good English (not all the time and we can get sloppy when relaxed) is when you get out in the business world. A well-spoken person always stands out and sounds much more educated than they actually may be.

How come is never acceptable in English English.

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There's nothing wrong with how come? It is somehat colloquial, which isn't a problem.