Yes, that's a somewhat confusing topic. It's best not to overthink it. Better to look at lots of examples in all contexts and let your brain develop an intuitive feeling for this. Then come back to the lesson and read it again. It will slowly start to make sense. But what's even more important, you'll be able to use it correctly -- even without knowing why.

What really helps with some of the more difficult-to-learn lessons like this is being able to test-drill your understanding of them over and over.  

Unfortunately, the only way to do this right now in KwizQ, according to the IT Helpdesk, is to have only one or two lessons saved in your Notebook and take the overall test (not the 2 Question lesson test) over and over again. This will provide up to 6 questions (if one lesson) or 3 questions (if two lessons) on each lesson topic at a time.  

But it comes at the very inconvenient cost of ditching any other lessons you have saved in your Notebook.  

IMO it would be far better if we could save all the lessons we want to find/refer back to/ revise easily AND be able to drill specific lessons whenever we wish.  We know learning is never linear, esp when at the early stages of still building your language-understanding structure. 

Après avoir lu tous les commentaires, je comprends mieux pourquoi les Anglais /Australiens ont tellement de problèmes avec ce point de grammaire. Bon courage à tous!