"Le garçon a un an."
Something like this (a verb being used in its' non-definitional way) could use a small footnote. eg, in french, the verb avoir is used to show posession of one's time.
The way I see it, this is not a teaching tool whatsoever. It's a self-testing tool after you've learned some using other sources.
I'm enjoying the program quite a lot and finding it very useful, but I agree, if I didn't already have a bit of a base in French, I think I would get frustrated pretty quickly.
Duolingo is about learning by experiencing: you don't need the footnote anymore, you know the rule now.
Something like what is displayed when you use il est or c'est incorrectly would be nice though for when you get the answer wrong.