Brokeback Mountain alternates between the private lives of our two cowboys, Jack Twist and Ennis Del Mar. The movie opens with the two men together, but they soon split up, and we alternate between Ennis and Jack, with more time being devoted to Ennis's complex relationship with Alma.
The film spans a few decades, and the postcards between the two men, each stamped with the date, progress the time and the plot. It's mostly linear, except for one flashback during the film's climactic argument. This flashback reminds us of when the men were happier, which was only about an hour ago for audiences, but almost twenty years for Jack and Ennis.
Because the men are our two POV characters, we never see the wives if the men aren't present. Also, Jack's death isn't shown, so we're left with Ennis's assumption on how Jack really died. Ennis is the type of guy who would fear the worst, but our characters are always reliable, so sadly, he is probably right.