Translation:She is always late. What's the matter?
I feel like В чём дело? is a stable phrase, and it's hard for me to break it apart and understand it as the sum of it's parts. Why does this construction mean "what's the matter?" exactly?
First of all, I'd say there are several stable phrases with this meaning: В чём дело?, Что случилось?, Ну, что? The difference is in formality, emotional state. I have a feeling that "Что случилось" is the most polite, "Ну, что" the least polite and "В чём дело" is somewhat in the middle, but they are all widely used.
The key word дело can be translated as matter, point, concern, business: ближе к делу = come to the point, get down to business, говорить по делу = to speak on business, дело не в этом = that's not the point.
It is an idiomatic expression, you can think of it as the matter consists of something. So the question is What does it consist of?. One can even ask В чём состоит дело?, which is the longer version and gives a hint. The answer follows the same pattern: Дело в том, что я беременна = The problem is that I'm pregnant [The problem consists of the fact that I'm pregnant]
Hope this helps, probably it would be easier just to memorize the whole phrase. =)
This helps a great deal, thanks! Should a longer answer to В чём дело? be in prepositional case?
Yes, one should say Дело в <предложный падеж>:
всё дело в машине [им: машина] = it's the car, дело в твоём поведении [им: поведение] = it's about your behavior.
You can always use this form, longer but grammatically simpler: дело в том, что <предложение без изменений>:
дело в том, что я хочу кушать = the problem is that I'm hungry.
Tagged me as having a typo for not capitalizing "she's" but a capitalized box was not offered.
This couldn't have been the case because capitalization isn't taken into account when marking your answer right or wrong. You probably had some other mistake in your answer.
So is the context for "В чем дело?" sort of like "what's the big deal?" or sort of a sarcastic "is it a surprise?" implying that her lateness is something that happens so often that it shouldn't cause any issues this time since it hasn't in the past?
I wouldn't say so. What's the matter? is the closest translation here. This phrase can be easily said by the boss or the colleague, so the context is like "Does she know about discipline?".
If I wanted to say "What's the big deal?", I would say Ну и что такого?, or А ты не знал? for "Is it a surprise?"
See higher up this discussion to see that "В чём дело?" is a fixed expression.