I recently got a PS3 and I have played the games I was interested in, but I don't like many games. Good thing there is a demo. I have a question. When you download a PS3 demo game that states 1 time trial demo exclusive, after the demo is finished can I play it again and/or download it again or is it a 1 time thing? Same question with 1 demo. Thanks.
You mean the PSN+ 1 hour trials? Those download the entire game (several GB) to your HDD, and then you have 1 hour to do as much as you can in the game. After that you can't access the game unless you purchase the full game.
Regular demos download only usually 1 level (usually anywhere from a few hundred MB to a couple GB) and you can play them as many times as you want.
Do I need a PSN+ account to do that? Also for the 1 hour demo, if suppose the 1 hour finishes and then the message appears "you must buy the game", can I delete the demo and download the demo again?
The 1 hour trials are PSN+ exclusive, so yes you need PSN+ if you want to try those. I don't think they would work simply by redownloading them, considering it uses the main game save file (so you don't have to start over if you buy it), because if that worked you'd be able to play the full game by redownloading it a bunch of times. They're also so big they wouldn't be worth redownloading. Most are in the 6-7 GB range. Some are more, like the Uncharted games at around 20GB.