Is an answer to a question an exception to the complete-sentence rule?

English grammar question: If I ask you a question, such as, “What color is the sun?” and you answer, “Yellow,” is that answer a grammatically correct sentence, even though it lacks a verb? Is it possible for an incomplete sentence to be a grammatically correct one?

(My brother posed this – his son, in elementary school, got points taken off an English test for answering with an incomplete sentence. And there wasn’t space on the test paper to write out a complete sentence anyway.)

Yes.

Definitely. The shorter the answer, the better. No need for verbs in some sentences.

WAG that because it is a direct, factual response to a posed question, the questioner really provides the verb and subject (in your example “The sun is”) for the explicitly given adjective making up the one word answer (“yellow.”). The questioner and answerer both understand what “yellow” is referring to in context, and so the assumed “the sun is” is unnecessary.

I’d say that in context with the cue of the questioner and the mutual understanding of both parties, “yellow” is a complete sentence. It is known that “yellow” is the response to the sun’s color. But without the contextual agreement between the conversationalists, it’s just a word (or if a single word may be called a sentence fragment, a fragment).

The teacher’s a witch for marking the kid down answering “Yellow” as not being a complete sentence, as the entire point of language is to convey necessary information, stuffy well-meaning but stupid rules be damned, and the old bat clearly knew the kid was answering the question with the relevant information (as he was marked down only for not using a complete sentence, not for his answer itself). I praise the kid for his conciseness.

Well, you can easily have grammatically correct sentences by using the command form of verbs. Example: “Take this.”

I seem to recall that this was acceptable because the subject was implied. Isn’t the same subject implied with an answer to a question? “What color is my shirt?” “[It is] green.”

I guess I’m the only one who was taught differently.

In a command, a subject is not needed, but a verb is. The subject is the “understood you”. “[You] Turn off the TV, please.” or “[You] Give the ball to your brother.”

But in a declarative sentance, a subject and a verb are both needed. There is no “implied subject” or “implied verb”.

It’s very common for teachers to require answers in complete sentances. The test isn’t testing only the knowledge of the subject, but the ability to communicate that knowledge effectively, which includes proper grammar.

Were it otherwise, the test answer sheet would simply be a list:

Green
Wombat
Curiouser and curiouser
42

Often, we use what is called an assumed (but missing) part of speech. For example, in the sentence “Never” is always an adverb. the word never is not the subject. The subject is the assumed phrase “The word.” If it were written out, it would look like (The word) “never” is always an adverb.

If you say Wake up!, or Get out!, it is not an incomplete sentence. The word “you” is assumed.

None of what I have written is going to change a test score, of course.

I suspect “yellow” is the wrong answer, but not for grammatical reasons. According to
http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

and

I’m not sure why that link doesn’t work: here it is again

Unless the purpose of the question was to elicit whole sentence answers. My writing intensive German language class required us to always answer in complete correct sentences.

Yes, clipped, elided, or otherwise coherent incomplete subject-verb-object sentences can be correct in many settings.

But certainly not on an elementary school test, where the purpose is obviously building the correct use of English.

In language, as elsewhere, context is everything.