when to use que, qui, ce que, ce qui

Unfortunately your proposed rule is incorrect. I'm not sure that I can explain it any better than the lesson, though. Maybe re-read the lesson and do some kwizzes? See if it clicks after a while. 

I suppose you are thinking about these two cases:

Mes plantes, que j'arrose tous les jours, sont très belles.

My plants, which I water every day, are very beautiful.

J'aime les bananes, ce que tu trouves fascinant.

I love bananas, which you find fascinating.

You can almost replace "which" with "that" in the first example, but it doesn't sound natural to me. (But perhaps you find this in poetry.) On the other hand, you definitely can't replace it with "that" in the second example without changing the meaning. So if you consider "that" acceptable in the first case, then the rule probably will work for you.

Hi Alan, interesting point about which and that in English:

My plants that I water each day... This implies that I have plants, of which I water only some each day.

My plants, which I water each day... In this case, I water all my plants each day.

This difference between inclusive and exclusive relative pronoun is an interesting and often overlooked distinction between "which" and "that".

You're right, in general, about the distinction, but the first example doesn't really work. You can't combine "my" and "that", in the way you suggest.  You'd probably just say "the plants that I water each day ...". If you insist on using "my", you need to say something like "those of my plants that I water each day...". 

So your first example would have to be interpreted the same as your second example, even though it's unusual to use "that" instead of "which" in a non-restrictive clause. 

On the other hand, it's perfectly acceptable (at least in British English) to use "which" instead of "that" in a restrictive clause - "the plants that/which I water every day are growing quicker than the others".

Yes, because "my plants" suggests an entire, undivided group, and a restrictive "that" clashes with this idea. If you had "the plants" instead, both which and that would work.

The plants, which I water every day, are beautiful.
The plants that I water every day are beautiful.

Here is a nice summary: https://www.grammarly.com/blog/which-vs-that/