The following contains spoilers for The Legend of Tarzan, as well as the books the film is based on.
First published in 1912, Tarzan is one of the most continuously popular heroes in literature. He's been the subject of movies, TV shows, cartoons, and pretty much any other medium you can conceive of. After a significant absence, the character has finally returned to the big screen in The Legend of Tarzan. While this is generally the character that most fans are at least familiar with, the new movie has taken considerable liberties with the original material.
While The Legend of Tarzan is not a direct adaptation of any of the books by Edgar Rice Burroughs, much less the first one, the film still contains a considerable amount of origin story, in the form of flashbacks and expository dialogue. We've pieced it all together to show you the significant changes between this Tarzan, and the original one.
In The Legend of Tarzan, we see Lord and Lady Clayton marooned in Africa with their infant son, John Clayton III. In the film, after the apes kill Tarzan's father, Kala adopts the baby for seemingly no explainable reason, beyond possibly "he looks interesting, and vaguely primate" and raises him alongside her own son, who becomes Tarzan's brother. Originally, Kala took in the infant after her own baby had died, so Tarzan becomes a surrogate child for the ape. It at least makes storytelling sense, if not biologic sense. The dead ape is left in the Greystoke crib, leading people to eventually believe John Clayton III is dead.
In the film, after Tarzan's birth parents are dead, the apes commandeer the shelter they built for their own uses. The cabin is a very important part of the story in the books, but for very different reasons. Tarzan eventually discovers the cabin on his own, makes his way inside, and begins to educate himself about the ways of man with what he finds. He's actually able to teach himself to read English with the books he finds (because, as a human, he's naturally more intelligent than the apes). He also finds his father's journal, which makes an appearance in the film as well, though in the book he can't read it, because his father wrote in French.
Life With the Apes
We don't see quite as much of Tarzan's adult life with the apes in the film, but what we do see is reasonably different. It's implied in the film that Tarzan leaves his ape family after he is nearly killed protecting Jane. While the two do meet in a similar fashion in the novel, by the time that happens Tarzan has successfully become king of the apes by killing the previous leader. He also kills Terkoz, the ape that attacks Jane. Also a random gorilla. And a lion. Tarzan just pretty much kills anything that gets in his way.
How Jane Came to Africa
The story of how Jane Porter and her father came to Africa is significantly changed from the books. In the film, Jane is a young girl who spent many years on the continent with her father while he taught English to the local tribe. In the Edgar Rice Burroughs story, Jane's father is part of a party that's searching for lost treasure, and Jane is along for the ride. They get marooned there much like Tarzan's parents were. They don't spend nearly as long in Africa in the book as they apparently did in the film. On the plus side, Margot Robbie's Jane is not the entirely useless creature that Burroughs version is.
The African Tribe
Quite possibly the least surprising change from book to film was in the depiction of the African tribes. As one might imagine, in the early 1900s, they were not treated well. The first Tarzan book only contains a single tribe. They are most similar to the one in the film that wants Tarzan dead. They are, in fact, lead by a man named Mbonga, as in the film. Also, Mbonga's son was killed by Tarzan after the son killed Tarzan's ape mother, though in the book Tarzan strangles the tribesman with a noose that Tarzan fashioned himself (again, because Tarzan is naturally gifted due to him being a white man). Also, in the book the tribe is cannibalistic. Yeah, some things had to change to avoid being terrible. There is no peaceful tribe learning English in the novel, all Africans are painted with the same brush.