You work is saved automatically every few seconds or any time you make a change to an answer, either by typing or deleting text or when you submit an answer to a multiple-choice question. Work is also saved when you click either Save or Save & Close at the bottom of an assignment.
Your instructor can always see work that's been saved. This means that, in essence, your work is "submitted" on an ongoing basis. Once you've completed an assignment, then there's nothing else you need to do to "submit" the assignment. It's already submitted!
This also means that as long as an assignment is set to Accepting Responses, then you're able to edit your answers at any time.
The one exception to this are locked sections.