If someone asks, "¿Es importante?", is it correct to answer with "No es"?
"ser" is a copulative verb and, as such, it usually requires a subject complement (SC). There are special cases, where "ser" means "existir", in which the SC will not be required:
Ser o no ser, ésa es la cuestión.
The SC can be an adjective or adjective phrase (AP), or a noun or noun phrase (NP). When the SC is an AP, omitting the SC will not be grammatical, and -- if it is not to be repeated -- the SC has to be replaced with "lo":
Sí, lo es. // No, no lo es.
I think that the only case in which an AP can be omitted as a SC is in echo questions like the one asked by C below:
A: ¿Es importante?
B: No, no es importante.
C: ¿Cómo que no es? (How can you possibly say it's not important?) Sí que es importante, y muy.
Now, when the SC is a NP, we have at least two possibilities:
- The NP merely describes the subject.
- The NP identifies or designates the subject.
In the case of (1), the SC cannot be omitted in the answer (except in echo questions like the one mentioned above):
Sí, lo es. // No, no lo es.
In the case of (2), the SC can be omitted and we may find "es" (or "no es") with a "truncated" SC (I'm not saying that this is obligatory, but just an option):
¿Estás seguro de que ésa es María?
Sí, es. // Para mí es.
We can even find this ellipsis of SC with common nouns that designate a thing or animal:
¿Esto es oro? No, no es.
¿Eso es un perro? Para mí no es.
In all of the cases with NP above we can of course repeat the NP or use, as other members proposed, just "Sí" or "No", but I thought it necessary to clarify that the cases of SC ellipsis above are possible and grammatical in the language.
Additionally to the example in the interrogative provided by pablodf76 (¿Es o no es?), the only other case I can think of in which "es" is used alone in a question is the colloquial:
¿Es o se hace? (meaning: Is he really a jerk or is he just acting like one/pretending to be one?)
We can also use it in the second person. In my country, where we use "vos", we frequently challenge an opponent by asking him (or her) this rhetorical question:
¿Sos o te hacés?
I don't think that it is incorrect, but the answer is kind of incomplete.
- You can answer directly with a simple: no
- You can answer using an object pronoun: no lo es
- You can answer in a diplomatic way reaffirming and using an object pronoun: no, no lo es
Any of these answers could be used depending on the context, maybe the third is the more polite.
Grammatically, it is correct. Pragmatically, it is not. The only real-life situation where "No es" would be totally OK as a reply to a question would be if the question was of the form "¿Es o no es?".
Suppose you're at a friend's house and he says he's got a book that may be yours. You don't remember having lent him the book, so you suggest it may be someone else's. You hesitate, so your friend loses his patience and demands to know:
Your friend: —¿Es o no es tuyo?
You, after thinking hard for a while: —No es.
And even then it sounds a bit odd because, regardless of the fact that a question posing two alternatives should not take a "yes" or "no" for an answer but only one of the alternatives, in actual conversations many people will start with "yes" or "no" anyway.