can u plz explain

Its a little bit confusing but lets begin with in this way. Whatever is going to happen to Harry now, has already happened in the future(coz it is future). Harry is going to go to past in an hour but the Harry in future is in his present. The Harry in future is in his past now and he is doing the Patronus charm to drive away the dementors. Our Harry (the present one) will do the same when he will go in his past to save harry of the past. I know it may confuse you but this is the only way to understand it.

This is really hard. I'm also confused with that. How can Harry know that his life was in danger that time when he is still in the present. If he was kissed by the dementors how could he save himself. I'm just really confused with that time turner thing.

Well, unless you ponder DEEP into it, it will be very hard for you to understand it. As you say, Harry fails to fight off the dementors and becomes senseless and all. However, when he and hermione travel back in time, they JUST travel back in time, they don't go back in their action. So, they become different from the ACTUAL Harry and Hermione of the past. They co-exist with the ACTUAL Harry and Hermione until the latter use the Time Turner to become THEM!!!! Now, do you get the point? Read twice or thrice more and you will become clearer. For a short period time in that night, there were two Harrys and two Hermiones. One pair was formed from the other, while the one who formed the pair blended into itself after using the Time Turner.

this part has ALWAYS confused me too. But I guess everything that was happening in that part had already happened...and Harry and Hermoine had already gone back in time...O.o I dont really know either, very confusing part! lol