In the "Assignments" tab, inside a group panel, you will find two categories:
Active: here you will find the assignments you have not completed or sent to your teacher yet.
Submitted: here you will find the assignments you have already sent to your teacher. These assignments cannot be modified.
Remember that if you complete an assignment but do not click on "Submit", your teacher will not receive it and will not be able to mark it. When an assignment has been correctly sent, you will see this confirmation message.
The assignment will move to the "Submitted" category.