Yes, you can compose part of the story and then select save draft instead of publish on the second page of the submission process.
I don't know if this works better than composing elsewhere and then pasting, which is what I did with my first story. What do other people here do?
Yes, there is a Save Draft button. However, like Jocelyn, I do my writing elsewhere (sometimes MS Word, sometimes Google Docs, even LibreOffice once or twice) and then paste the final product into the submission box. I just don't like the editor here compared to tools like those.
But sometimes, on some systems, things can go horribly wrong formatting-wise if you try to paste from, say word, into a text box. Isn't it the case that stories here are supposed to be in plain text?
If any mods are reading this, sorry for the dumb questions but it would be really helpful to have a sticky post with advice on how to actually write a story on here, not just the advice on grammar and so on. It's not difficult though, even I managed to do it!
Just be aware that no one but you can read it until it's been verified and posted.
If you want feedback, that only option I can think of is to send the file (possibly in Word) to people you know.