I just tried "ref" and was told no… :)
Ref is not really a word, anyway, just an abbreviation of a word. Still, the program is not consistent about whether its goal is to help you learn Greek or to help you learn English. :)
Sometimes an appeal is made to a standard of English, sometimes not, in their determinations of "correct" answers. The standard appealed to is often subjective in itself.
Sometimes mere demonstration of comprehension seems to be the basis for correctness.
Any any rate, I consider the best policy to make a comment, learn what the program wants me to say, and not worry about it.
This is actually a bit of a tricky question, because sometimes people might be trying to learn a language through one that they don't really speak, so the grammatical corrections are useful. But then they can be annoying to native speakers.
In an ideal world, DL would have a 'strict' mode where it did correct errors in both languages, and a 'lax' one where it was more generous in the language being learned from, but who do I see lining up to pay for that ...