Livre d'Or

OK, more on adjectives now I have my grammar in front of me:

It doesn't make a difference, that I can see, whether the adjective is predicative or attributive. Ett gender mostly declines, ie you add the -t suffix to the adjective in most cases. En gender either declines (by adding an -n suffix) or doesn't, and I think it's related to the spelling of the adjective itself but I'm not sure. Indefinites follow those rules, such as a, some, no, any. Plain definites also follow them. But definites such as this, that, possessives and the same randomly want plural adjectives. I suspect they're not really plural actually, because for the irregular adjectives this form looks different from a plain plural, though it's the same for the regular ones.

As you can see, I'm quite confused! I expect the answer is just familiarity, doing lots of reading until the right form just looks natural, and then coming back to think about what the underlying rules are.