How does this site work?

Instead of having to write numerous (and boring) explanations over and over (and over) again on your schoolwork, here's a somewhat easier way to go about it.

Your teacher wrote numbers on your assignments and these numbers refer to the numbers on this page.

Look up the mistake you made, look at the rule that goed with it (and, naturally) look at what it SHOULD be.

When you think you get it from the explanation offered, practise to see whether you really did get it with the exercise site or the exercise app. 

If it's not yet very clear to you, please go to the explanation offered in the video.

Then practise your skills with use of the site or app mentioned.