You can open it from the minute you start entering answers into an assignment.
It only shows text answers, with teacher’s comments if there are any.
You can copy the text of an assignment out of it at any time, to put into a word processor.
If your teacher wants you to have another go at an assignment, he or she can leave it unlocked after marking. That means you can go in and change it, then submit it to be remarked – hopefully at a higher level! Don’t forget to open the Summary window so you can see the teacher’s comments as you work.