I've seen there's a Finish task option on mi list of assigned tasks but I'm afraid if I click it I'll lose all task's submissions. Is there a way to keep them?
Yes, there is a Finish task option on all your tasks that have received a submission, but don't worry this won't affect your submissions in any way.
In the Assigned tasks page you will see a tab containing all the finished tasks so you can review anything you want.
And in the group page you will see the finished tasks at the end of the list and the submissions will appear as always.
So, don't worry your submissions are safe when finishing tasks.
Have a good day