Write a few lines and get a score plus a natural rewrite. Each change is explained one by one, so you see what was fixed and why.
Does it only mark what is wrong?
No. It shows how a native speaker would have written it, and when something is grammatical but still reads oddly it says why.
Is it fine to write only a little?
Two or three sentences is plenty. What matters is that the phrasing you actually reach for shows up, not the length.