Could be, but "Est-ce grave?" is more formal and will sound unnatural out of certain contexts, like say, academic meetings, political speeches, literature, etc. (as virtually all "subject - verb" inversions to form questions).
I find it really really good that DL uses that type of question forms, because everybody makes questions like that in everyday French, without it being vulgar or mediocre.
The "est-ce que...?" looks more formal because it has more words, seems more complicated; but it's in fact way more informal than the simple "subject/verb" inversion.
Keeping the regular shape of the sentence to make a question, as in "C'est grave?" (you know it's a question by the tone of the speaker, or by seeing the question mark if it's written), is very informal but actually it's the most used form in everyday, regular, French.
I think in English, using that form, i.e. "It is serious?", is way more colloquial than in French; whereas using the correct "verb/subject" or "do/does +.... ?" sounds totally natural in everyday English. In French, making inversions sounds really really formal, posh or... just foreign! lol (in a common, everyday context, of course).
Between those two extremes, you have the "Est-ce que...?", which is as correct as the rest, while being a good compromise between formal and informal. Plus, it's simple to use, just start with "est-ce que" then add the sentence as it is in the affirmative form.
I know what you mean, "grave" can mean "important", but without a context, and mostly in that form ("is it...?"), you'd never use it as "important", at least I can't think of examples where you would.
"C'est un sujet grave" = "It's a serious subject" or indeed, why not "It's an important issue", could be, although to me, there's a nuance. But still.
"C'est grave?" : you couldn't use that question, asking about that very issue in the previous example.
So, when would you ask that?
Talking with the doctor about medical results, or with the police, or anybody who's about to announce you something, etc. Then you wouldn't say in English "Is it important?", rather "is it serious?" for instance.
asking someone if what they're saying, what they're wondering, is... kind of "important", you're right! But you wouldn't ask, again, in English "Is it important?". Example:
"Je n'ai pas fermé la fenêtre en sortant!!!" --> "C'est grave?"
"I didn't close the window on my way out!!!" ---> "Does it matter?", or "Is it risky?", or "Is that a problem?", etc.
I'm not a native English speaker, but I don't think "Is it important?" would be a choice in that case...
uh... First, it would be nice if you could mind your spelling a little bit... :-)
"She's serious" could only be "Elle est sérieuse", meaning she's not kidding.
If you use "grave" with a person, i.e. "elle est grave", it's generally colloquial to mean "she's nuts, mental ! / she's something ! / she's audacious, bold, she dares !" - or maybe that she looks / sounds serious (but "elle est grave" is little likely to be heard in that sense).
Then, the French sentence is "C'est grave", and "c'est + adjective" can never be about a person, male or female. That c' comes from ce, ceci/cela, and it means "it, this or that".
Yes, "Is he serious?" as a translation to "C'est grave ?" is incorrect.
Even if this was a situation in which you could say "il est" or "c'est", that would not mean that you could use "he" in English. You precisely never translate "il" into "he" in that case, because it is an impersonal "il", as in:
il est 18h = IT is 6pm
il pleut = IT is raining
But besides, you cannot use "il" in this sentence here: "il est grave", although grammatically correct, does not make sense to mean "it is serious". At least, as it is. You would need something after "il est..." for that sentence to be correct:
- C'est grave de dire ce genre de choses = It is serious to tell that kind of things / Telling that kind of things is serious
- Il est grave de dire ce genre de choses : same meaning, but more formal
The only other case when you can use "c'est" or "il est" about people, is when used with a job:
- Il est médecin = C'est un médecin
Otherwise, using "il est" alone with just an adjective, would indeed refer to a "he". And you cannot really use "grave" to mean "serious" with a person; you'd then simply say "Il est sérieux" (or "Il a l'air grave" if you mean "he looks serious"). "Grave" with people is generally colloquial French, almost slang:
- Il est grave! = He is mental / he's got nerve / what a cheek! (or possibly, "He is low" talking about someone who's singing for instance)