What are the main differences between the Pride and Prejudice film and novel?

There are two major film adaptations of Pride and Prejudice: the 1940 version starring Greer Garson and Laurence Olivier, and the 2005 version starring Keira Knightley and Matthew Macfadyen. Since modern audiences are more likely to be familiar with the latter, this is the one addressed here.

The major structural difference between the film and the novel is that the film ends with a flash-forward to a married Elizabeth and Darcy living harmoniously at Pemberley, whereas the book concludes with their wedding. Darcy is also a much more romantic and less repressed figure in the film. This is emphasized by changing the setting of his two proposals from a drawing room and a street to a classical folly in the pouring rain and a misty moor.

Elizabeth's character has also been altered in the film to make her more romantic and less sensible, while the Bennets live in picturesque poverty, with their house in a state of disrepair. In the novel, they are wealthier for the time being, even though threatened by poverty in the event of Mr. Bennet's death.

What are the main differences between the screen version and the novel of Pride and Prejudice?

[This question is too broad for eNotes format so only a brief overview can be provided.]

There are now many filmed version of Pride and Prejudice (including the Bollywood phenomenon Bride and Prejudice). Since you specify no particular version, I'll speak of one or two as they come to mind as examples.

Of course, the primary difference between Austen's written original and filmed adaptations or interpretations is that filmed versions have camera angels, music, acting and acting choices (as well as directors and directorial choices), with alterations to subplot emphasis, lines and scenes, and settings. What is written by Austen is fixed and unvarying. What is filmed is fluid and variable.

To illustrate this fluidity, while Austen presents Elizabeth and the Bennets as an upper class country gentleman's family (though Mrs. Bennet nee Gardener had been middle class like her brother) with servants and farm workers ("Mrs. Bennet, who assured him with some asperity that they were very well able to keep a good cook"; "your father cannot spare the horses, I am sure. They are wanted in the farm ...."), Joe Wright's 2005 version with Keira Knightley presents Elizabeth and by extension the whole Bennet family as uncouth and ill-mannered country "folk." This is made clear when Keira has Elizabeth defy civil manners and Lady de Bourgh by shoving the whole soup spoon into her mouth instead of taking her soup from the side of the spoon. This illustrates the fluidity of the film versions while also illustrating the effect of acting choices and directorial decisions.

The most profound differences between the original and the filmed versions is the loss of the narrator and the addition of camera angles. The narrator is removed from each filmed version made to date, with important narratorial lines being given to various characters, often to Elizabeth, but not exclusively. This robs the film scripts of the objective view of sincere society that is realistic, and not a caricature of society, not a parody of society, that Austen's involved ironic narrator provides. Thus characterizations are altered toward parody and farce, as is seen most often in the character of Mrs. Bennet. The addition of camera angles given an opportunity for delving into emotions and psychological motivations which assists in developing intricate character reactions that would otherwise be explained by the narrator. Simon Langton's 1995 BBC series with Colin Firth illustrates this when the low-angle mid-range shot shows Firth's rendering of Darcy's reaction at seeing Wickham in Meryton.