The story, if you've forgotten, tells of Robert Jordan, an American member of the Abraham Lincoln Brigade fighting fascist Loyalist forces in the 1930s. He's a Spanish professor from Montana, so he's fluent in the language and somewhere he's picked up knowledge about how to make bombs. His task is to blow up a bridge during an attack by Republican forces, so the Loyalist ones will be trapped and wiped out.
Easier said than done: he works with a group of guerilla fighters, who have various talents and experiences. A beautiful 19 year old girl who'd been raped repeatedly by the Loyalist is among them. She's been traumatized but nevertheless they fall in love and have three days together before almost everything ends badly.
Hemingway writes powerfully and very clearly about what this sort of war is like: I was reminded of Tim O'Brien's masterful fictions from the Vietnam War. Much ugliness is there, with few heroes and a great deal of lies. Anyone who knows Hemingway's own story can also read many hints of exactly how his own drama will end.
But Hemingway has chosen to directly "translate" the Spanish spoken by Jordan and his partisans into English, particularly at the beginning. When the friends or lovers address each other they frequently use the "Thou," the second person singular form which hasn't been used in English for a couple of hundred years. The result is either comic, awkward, or read from this distance, a shocking case of condescending appropriation of voice. A reader of the original in 2019 might well be tempted to throw the book aside after a few chapters.
However in French, the second person singular is used all the time among friends, family and lovers, so that infelicity falls away because addressing someone as "tu" is commonplace. The story shines through, and it is worth reading.
My advice to readers of the original: grit your (proper second person plural, you'll notice) and carry on. Although I must note that apparently when I'd read the novel at about age 19 I only read the steamy love scenes: there was much that was new to me.