That's a good and tough question. In the beginning of the novel, Candide simply learns that the world is not what he had thought it was—that it was more, and more rugged. He starts naïve, and sees the world.
He then learns fate and mercy, when he is forced into the military, and then saved from a whipping.
He learns that there are more complex ways to view the world when he talks with the philosopher, and, more generally, that there are many views of the world.
His final lesson, which may not be the book's lesson, comes in the end of the novel, where he learns that this is, as Pangloss taught, the best of all possible worlds, and that we must all "cultivate our garden."
i think that candide throught the book learns that there are many diffucult situations and that it is not always for the best like pangloss says.
Through the book, Candide learns that all is not for the best. In the beginning, he thinks that Pangloss's idea of "all is for the best" is true, but as he suffers more and goes through more difficulties, he starts to change is mind and thinks that what he is experiencing is not the best thing that could happen.