"The mouse is small."
Translation:De muis is klein.
Apparently the adjective drops the e when it comes after the verb. This is not mentioned in the Tips and Hints
You can also go to the Adjective Basics page (the one that lists the lessons) on the web and scroll down a bit to see a "Tips and notes" section with tons of useful information about when to add the 'e'.
You can also access this text if you're on the web by clicking the subtle, grey "Tips and notes" link in the upper left corner while you're working on an "Adjective Basics" lesson.