I was watching The Disappearance of Suzumiya Haruhi, and the scene when Asakura attack Kyon is rather confusing. So this is what happens:
After Kyon has a mental struggle with himself, when he was about to shoot normal-Nagato with the device that android-Nagato gave to him, he suddenly got attacked by Asakura. She stabbed him twice, and when Asakura was about to kill him, suddenly Nagato (perhaps the android one?) saved him. And when he was about to lose consciousness, young and old Asahina come. And then someone (perhaps future Kyon?) comes. After that he loses consciousness.
Later, he woke up at the hospital with Koizumi. And when Kyon asked what happen, he said he fell down the stairs at school. But Kyon seems to know what really happened before.
If I have to guess, this is what I think happened:
After Kyon recovers from his injury, he, Asahina, and android-Nagato travel back to the past, and save the past Kyon after he got stabbed, and Nagato alters some memories about the incident so Koizumi didn't know anything about it, and they travel back to the current time.
If that's what happened, why he didn't save himself back then before he got stabbed? What's the point of giving an antidote for Nagato to Kyon? And why did Nagato give false memories about the incident to Koizumi? What exactly happened here?