One of the major reasons why Faulkner can be so hard to read is his use of long run-on sentences like, say, this one:
Knows remembers believes a corridor in a big long garbled cold echoing building of dark red brick sootbleakened by more chimneys than its own, set in a grassless cinderstrewnpacked compound surrounded by smoking factory purlieus and enclosed by a ten foot steel-and-wire fence like a penitentiary or a zoo, where in random erratic surges, with sparrowlike childtrebling, orphans in identical and uniform blue denim in and out of remembering but in knowing constant as the bleak walls, the bleak windows where in rain soot from the yearly adjacenting chimneys streaked like black tears. (6.1)
Yeah, we didn't think your teacher would like that one either. This paragraph-sentence also features other Faulkner traits such as compound words ("cinderstrewnpacked," "sootbleakened"), repetition ("bleak walls and bleak windows"), and made-up words known as neologisms ("adjacenting"). While at first the paragraph might seem completely overwhelming, it helps to keep in mind what's being described – Joe Christmas's memory. And how do people remember? In bits and pieces, and in flashes of information that may not make sense on their own, but when put together they create an emotionally complex collage of the past.
Another feature of Faulkner's writing is his use of perspective. Since this is a tale of a local community, the narrative jumps around from Lena's perspective to Byron's, from Joe Christmas to Reverend Hightower.