This is why don't normally read up on grammar specifically and just get a general overview and build up understanding through reading, or in this instance just asking about it, because more often than not it just confuses me and alot of the time really makes no difference :P
That explanation from your book is the kind of explanation i just stop reading half way through lol, just because i stop and think, "Hey, if i didn't need explanations like this as a kid to understand english then i don't need explanations like this to learn japanese"
