When someone says a sentence "sounds unnatural," it doesn't necessarily mean it's plain and simple wrong. Instead, it typically means: - The sentence is grammatically correct, but the wording, phrasing, or structure doesn’t sound like how a native speaker would naturally say it. - It might feel awkward, overly formal, too literal, or influenced by another language’s structure. Tip: "Unnatural" sentences can usually be improved by adjusting word order, idiomatic expressions, or tone to match common usage in the language.

It may not be wrong, it might just be a manner of speaking that is not very natural. Natural: Would you like to get a drink? Unnatural: Do you have a desire to ingest liquids for hydration? That's an extreme example, but hopefully it gives you the idea. There are no errors in the 2nd sentence, but nobody would ever say that.

I'm not sure there's a precise answer to this question. Some sentences that sound unnatural are plainly ungrammatical, or use incorrect vocabulary. Some sentences that sound unnatural use acceptable grammar or word choice, but depart from the normal practice of native speakers, and thus sound, well, unnatural. It depends! And even in the second case there are *degrees* of unnaturalness: some sentences sound just a bit awkward or unnatural, and others dramatically so. I think context really is the key here. Sorry this isn't more straightforward!

Generally it means that what you want to express is more or less understandable but the way you have said it is strange or unusual.

It means that the sentence doesn’t sound natural. It isn’t how a native speaker would phrase the sentence.