Can you please define "usable sentence?"
Can you please give me an example of a usable sentence?
Thanks for your help.
What are you writing about? If you are writing an essay about the pros and cons of something, but you include a sentence about something completely off-topic, I'd call that NOT a usable sentence.
However, since I have no idea what you mean by "usable sentence," that is just my guess.
Please clarify.