What the hell is copyright?

It is understandable that copyright can protect people's descriptions of their idea whether on paper or anything else. But how does it specifically protect it? For example, someone learned grammar, vocabulary etc. and how to form neat sentences from books. The books have copyright, and eventually this person that learned from that book might somewhere use a very similar expression, forming of sentence or even word to word to another idea. Where does copyright lie here? Can't someone read a novel and memorise a lot of sentences and use them time to time because they eventually become "his" words? It is nice that copyright does protect novels etc. but what does it actually mean that you can't physically copy something because it's the creator's own original work? What original work is this about? Many sentences are alike and people that are learning literacy at school are learning to write the same way as the best author's in the world. Obviously when you write something it will look different to an extent. For example, from a newspaper I'm quoting a sentence "But when asked about that pledge her office was quick to clarify." can't this sentence ever be used word to word again because the article is "copyrighted"??? Please can someone clarify...