Obi wan first met Anakin as an adult during the clone wars, and he was amazed at how strong the force was with him. He decided to take it upon himself to train him as a Jedi, just as Yoda had trained him in the past, NOT by Qui Gon Jin, who didn't bloody exist. He thought he could instruct as well as Yoda, but Anakin eventually went over to the dark side because Obi Wan wasn't nearly as good an instructor as he thought.
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