5Struggling with the ar
cticles, I'm close to admitting defeat. So am I - and I am a native speaker with many years of teaching experience!
1. The rivers of Britain are short. (There is no context. This is a sentence from an exercise book. There're lots of rivers in Britain, why 'the'?). All the rivers of Britain. As a group, they are as unique as 'the sun'.
2. Rivers in Britain are short. (I see almost no difference if compared to N1. Just rivers in Britain; it's not important to identify which. Or should the sentence begin with 'the rivers'?). It's 'in' that's important. I imagine that you are OK with 'Rivers can be difficult to cross' or 'Rivers are wide, but streams are narrow'. So, - 'Rivers in Britain are short.'
3. John has a wife, a son and three daughters. So, John is the head of a big family. (here 'a big family' acts as a category, I understand that. But is it also okay to say "the head of the big family' - his family?). Not unless we have been talking about several families, a big one, a small one, a disfunctional one, etc. Then, after the first mention, we can speak of the big family - we know which one we are referring to.