Is a single adjective proper grammar? [duplicate]

Would something like

Cold and soaked, she waits in the rain. Forgotten.

be proper English? Specifically the "Forgotten." part

I know sentences need to have a subject and a verb, but we also have sentences like this

Are you going?

No.

There is no subject or verb in "No.", and yet that is still a sentence, isn't it?

In short, no, 'Forgotten.' all by itself is not considered a sentence and so is not grammatical as a sentence. Or if this is a question about acceptable style, single word utterances tend to the too informal, and therefore is not 'proper' either.

But, this is not so straightforward as it would seem. Presumably a sentence is either correct or it isn't - that's how correctness works. And that works in most situations. But sometimes there are cases near the boundary where it is a bit fuzzy.

By most accounts, a sentence is the only form that is considered grammatical/'correct' or not. A sentence is defined by having at least a subject and an object. A single word utterance can't have both. By all that is logical, this precludes a single word from being a full sentence.

Of course, there are exceptions given in some situations. "Lo!", "Hark!", "Hello!" are considered well-formed sentences in English. To the question "What color is that bird?", the answer of a single adjective "Red." is as acceptable as anything else in proper English.

However, people communicate information and it is structured with pattern. Some literary styles are detailed and complicated and like a sentence from Jane Austin. Others are elided and terse, like Hemingway. Others might try to capture all the peculiarities of actual people talking like Damon Runyon. It is all still rule-based ('grammatical'), and all still acceptable and natural to its intended audience.

Cold and soaked, she waits in the rain. Forgotten.

This is perfectly acceptable literary style of a certain kind. A high school grammar teacher may balk at it, but it's perfectly acceptable as fiction writing. If it were writing for a newspaper or repeated more than once, the newspaper editor might have the writer fired or the publisher might roll their eyes for using such a clipped style. But readers wouldn't think this is wrong or incorrect or foreign sounding.

All I've said sounds like it is universal, but frankly all sorts of interpretations might hold in other languages and cultures. Very grammatical sentences might be expressed via what is a single word in many languages (Inuit and Turkish are prime examples). And a French writer may say this is absolutely forbidden (except when it's not).