She took me to her elfin grot, And there she wept, and sigh'd full sore, (lines 29-30)
After riding around on the knight's horse all day, the fairy lady invites the knight back to her "elfin grot," or cave. But why does she start crying and sighing? Is it because she regrets having sex with the knight? Or because she's sorry for what she's going to do to him? If she's feeling remorse, why does she abandon him, anyway? This is one of the most ambiguous lines of the poem.
And there I shut her wild wild eyes With kisses four. (lines 31-32)
Again, the fairy lady's eyes are described as "wild," and this time, the knight calls attention to this wildness through repetition. One of the fairy lady's defining characteristics is her "wild wild eyes," so it's important that the knight tries to neutralize them by "shut[ting]" them.
And there she lulled me asleep (line 33)
The sweet, almost maternal feel of this line conflicts with the anxiety the reader feels about what the fairy lady is going to do to the knight once he's asleep.