When you create a form you have a lot of different options, under the Settings tab. One thing you will, most likely, always want to do is select the "Collect Emails" option. This will allow you to see who submitted the form, even if you forget to add a name option or they don't fill in that question.
Other options you may want to choose would be to allow them to edit the form after submitting it, submitting more than one, or making the form a quiz.
When creating a quiz, you will select the "Quizzes" option under settings. The first step is to "Make this a quiz." From there you have different options. If you want the quiz to be "locked" the students will not be able to open any other tabs once they open the Form - if they do, they will be redirected.
You can also set the quiz to have an answer key - this is most beneficial for multiple choice. Different questions can have different point values as well. If you have some questions that are short answer, or that you would need to grade manually, you may want to choose the option of releasing their grade after you review it. I like this option since it automatically gives that question 0 points.
The easiest way to quickly see if you have this option is to create a Form, go into settings, click "Presentation" and see if you have the option to disable autosave. This means that your Forms will automatically save progress.
You can also tell that the Form is saved by looking for a cloud with a checkmark in it at the top of your screen.
Form progress is saved for 30 days. In order to access the saved Form, you simply click on the original tab. As long as you are signed in with the same email account that you started taking the Form, your progress with be saved.