What are the parts of speech of the words in 车外的阳光格外的刺眼, and why is this 的 correct?
车外： Object + related location/position = place term, n.
的： Aux. term to turn n. into descriptive, usually in possession/belonging relation; or habitually added after multi-character adj. (single character adj. usually is optional with 的)
阳光： n. subject of the sentence
格外： itself is adv. no need to add any aux term.  月亮格外明亮/汉语格外难学
的： Double mistake - no need at all, or even if you want to put aux term, it must be 地
刺： v. as predicate.，literally means &quot;to sting&quot;
眼： n. as object.

Perfect:  车外的阳光格外刺眼。
Not so good:  车外的阳光格外地刺眼。