In one novel I read recently (One Day-David Nicholls) the writer uses both present tense and past tense and I can't figure out how, where, why and if that is something that one should try, and to what purpose. When is that ok and how does one know when it should be one or the other? Actually I didn't notice it on the first read, so it's not like it bothered me, it was only now when looking through it again that I saw this. I also noticed that he seems to mix pov's in the same scene from time to time. does it mean that it is some kind of omniscient pov or just breaking "rules"?