plz help me! i'm puzzled.

For the same reason that, after Harry and Hermione use the Time Turner, they see themselves in Hagrid's cabin.

The easiest way I can think of to say this is like this:

The Time Turner takes people back in time. It doesn't erase the people that are already in that time (one person in two places at once). In this case, it created two Harrys and two Hermiones. One that had used the Time Turner and one that had not. One Harry - the one that had not used the time turner - was on the lake, being attacked por Dementors, the other had used the Time Turner and was on the other side of the lake, fending off the Dementors.

i know it is a bit strange and complicated but if you manage to change the past you change the future. it's a Vicious circle, the first time it all happens the same way as the seccond time, only they don't know yet that they will be the ones doing it. they do travel back in time so they are there at the same time as before they've gone back in time.

i hope you understand it this way but in the book the same story is told twice, only one time from them actually being there in normal time and one time as time travelers.

hope this helps

When Harry thinks he sees his father, it's actually his future-self. But he doesn't already know that because who should have told him that he and Hermione will travel back in time later?
So then, when he is at the lake again and sees his past-self being attacked por the dementors, he understands that nobody apart him is there to produce a patronus and that he has to do it.

Yeah, well the thing is that Harry Potter is still a sort of a fantasia book and you won't be able to find a proper explanation for all the phenomena coming up in the story. It's a bit like religion, Aloe Vera or psychology. Works out only if you believe in it.