"And there she lullèd me asleep, And there I dream'd – ah! woe betide! The latest dream I ever dream'd On the cold hill's side.
The fairy lady "lulls" the knight to sleep like a baby in her cave, and he starts to dream something.
He interrupts himself with a dash – in line 34, and exclaims "Ah! woe betide!" because even the memory of the dream is horrible as he repeats it to the unnamed speaker.
"Woe betide!" is an archaic exclamation used to express extreme grief or suffering. It was old-fashioned even when Keats was writing.
The knight's use of this expression emphasizes the medieval romance setting.
The knight's dream in the fairy cave is the "latest," or last, dream he'll ever have.
Stanza 10, Lines 37-40
"I saw pale kings and princes too, Pale warriors, death-pale were they all: They cried, 'La belle Dame sans Merci Hath thee in thrall!'
The knight describes the dream he had: he saw "kings," "princes," and "warriors, and they were all "death pale." In fact, he repeats the word "pale" three times in two lines.
This procession of "pale" men could be an allusion to the fourth horseman of the Apocalypse that gets described in the Book of Revelation in the Christian bible. The fourth horseman is Death, and he rides on a pale horse.
The pale warriors, princes, and kings all cry out in unison that "La belle dame sans merci" has the knight "in thrall," or in bondage.
Line 39 has the title of the poem in it, so it's time to translate it. The title is French and it translates to "the beautiful woman without mercy."
(If you want to know more about the title, go to the "What's Up With the Title?" section, and then come back.)