The hardest part of this book is probably the word "Abnegation," but you'll get used to it. Exposure therapy and all that jazz. Tris mostly tells her story in plain-jane prose. For instance, using our patented "open the book at any point" technique, we find:
Marcus walks up to Tobias and wraps his arms around his son. Tobias stays frozen, his arms at his sides and his face blank. I watch his Adam's apple bob up and down and his eyes lift to the ceiling.
"Son," sighs Marcus.
Tobias winces. (39.41-3)
Pretty straightforward, right? No one is going to mistake this for Faulkner's As I Lay Dying. The feelings here may be complex—Tobias/Four is seeing his abusive dad Marcus for the first time in a while and he's not too happy about it. (Huh, those feelings aren't so complex after all.) But even if the feelings are complex, the prose is pretty simple, like: Noun (Tobias) + Verb (winces). Okay, there are some harder, compound sentences to be found, but overall the writing shoots straight.
But here's one additional problem—because you're not lucky enough to have it that easy: we get all of this from Tris's point of view. So, do you trust her? Does she understand what's going on around her? It sometimes seems like the real difficulty here isn't understanding what Tris is saying (noun + verb), but understanding what she's leaving out or doesn't comprehend.