Does anybody know how to delete a course? Thanks!
Just be careful you don't accidentally delete a course you want to keep. That can't be undone.
On the web site: hover over your username. On the drop down menu click settings. Then click Learning Language. From there click reset or remove languages. Click the remove button next to the language you want to delete.
If you go to Settings, Learning Language, there are small grey words saying "Reset or remove languages." Click on the words and then you can delete languages, although it deletes all of your progress.
Here's a more thorough article from Duolingo's Help Center regarding how to delete a language from your profile --
I'm curious, why do you want to delete a course? Something you have tried for a little while as experiment and not really learning, or a tree you have actually put a lot of time in, such as your Spanish tree? The latter case would be kind of a shame, in my opinion.