Have you read the Tips & Notes? When you click on a Skill, you will see a light bulb next to the Start button. Click on the light bulb and you will get all the information you need for that Skill. If you still have questions, please return to the discussions and we'll help you out.
This exercise accepts "understands", "understood", and "will understand". For third person singular present you should use "understands". Rather than recognize you left off the "-s" from the present tense version, the Duolingo software instead assumed you left out the "will" for the future tense version.