Away! away! for I will fly to thee, Not charioted by Bacchus and his pards,
All this thinking about how depressing the world is makes the speaker think, "Get me outta here!" He needs to hatch an escape plan.
He wants fly away to join the nightingale in its refuge from the world. But he knows that the booze isn't going to take him. He can't rely on Bacchus, the Greek god of wine, or any of Bacchus's buddies ("pards"), which is what he wanted earlier in the poem.
But on the viewless wings of Poesy, Though the dull brain perplexes and retards:
Instead of wine, he's going to fly on the wings of his own poetry. Poetry's wings are invisible, or "viewless."
He's hopeful that poetry will take him to the nightingale's world even though his brain is not so helpful in making the trip. His brain confuses him and slows him down.
Already with thee! tender is the night, And haply the Queen-Moon is on her throne,
And, the, all of a sudden, he's with the nightingale. How did that happen? Count us slightly suspicious of how he can be "already with" the bird, even though he just complained about how his brain was such a big roadblock.
One possibility is that he joins the nightingale in his dreams, because the imagery in this section is associated with darkness and night.
He is in the kingdom of the night, which is soft and "tender," and the moon is visible in the sky. The imagery is more fanciful and imaginative here.
The phrase "tender is the night" was made famous by the American writer F. Scott Fitzgerald, who used it as the title of one of his novels. (To find out more about what it's about, check out our Shmoop guide to Tender is the Night.)
Cluster'd around by all her starry Fays; But here there is no light, Save what from heaven is with the breezes blown Through verdurous glooms and winding mossy ways.
The moon is surrounded by her attendants ("fays"), the stars. Despite all these sources of light, there is no light in the nightingale's world beyond what filters down through the trees.
What he is really describing in this complicated-sounding line is the fact that the nightingale lives in the forest, where trees block the light. "Verdurous glooms," just means the darkness that is caused by plants getting in the way of the moon.
Still, the nightingale's home sounds like a magical place, something out of a fairy tale.