Why did Obi-Wan say that Yoda was the one who instructed him when it was Qui-Gon Jinn that did?
This is explained in Attack Of The Clones. It is revealed that Yoda teaches Jedi-Younglings (young children) the basics of the Force and how to control it. After a certain amount of time, usually by the time the child is bordering on young-adult, they are promoted to the rank of Padawan and a Jedi Master takes them on for the completion of their training until they are granted the rank of Jedi Knight. So Obi-Wan would have been instructed by Yoda until he became a Padawan, then Qui-Gon Jinn would have been paired up with him.
