If you didn't manage to finish your topic or lesson, you can continue with them at any time from your Activity page!
To resume a topic:
Go to your Activity page to find all of the topics you've recently worked on. Once you've found the topic you're after, click "Resume" 😀
To resume a lesson:
Go to your Activity page and click "Check results" on the topic that you were working on. You'll then see the "Resume Lesson" button to continue with your lesson:
Done! Now you can carry on from where you left off 🎉