With tremulous cadence slow, and bring The eternal note of sadness in. (13-14)
We've pointed this line out in a few other places, because we think the end of this stanza is such a crucial point in the poem. It's where we really start to shift gears, and move from a calm little poem to the grand questions of life and eternity. It's worth pointing out that these are lines 13 and 14, which would be the last two rhyming lines of a sonnet (you know, if this were a sonnet). Here, instead of tying the stanza off with a neat little bow, Arnold raises big, kinda scary questions. It's just another way he's messing with tradition in this poem.
Sophocles long ago Heard it on the Ægean, and it brought Into his mind the turbid ebb and flow Of human misery; we (15-18)
One thing that has made the works of Sophocles last for thousands of years is his ability to tackle big themes in ways that still seem relevant. If his writing about human life and existence didn't still seem meaningful, we wouldn't still care about plays like Antigone. That's why Arnold name-checks him here, because he thinks they both care about the big issues. It might seem a little pretentious, if "Dover Beach" weren't such a good poem, IOHO.
Of human misery; we Find also in the sound a thought, Hearing it by this distant northern sea. (18-20)
The sound of the waves on the English Channel calls up a thought in the speaker's mind. Well, actually it seems like it brings up a whole bunch of thoughts about the nature of human existence. This poem is all about how paying close attention to the natural world can awaken grand philosophical thoughts in our minds.
To one another! for the world, which seems To lie before us like a land of dreams, (30)
The notion that the speaker puts forth here, that the world is a dream, and happiness is an illusion, has a long philosophical history, from Plato to The Matrix. In every case, we need a Morpheus to wake us up to reality, to show us the "desert of the real." The speaker of "Dover Beach" is doing that for us, waking us up to the ugly emptiness beneath the illusion of reality.
And we are here as on a darkling plain (35)
We just love the contrast between the beauty of these words and the darkness of their message. Doesn't "darkling plain" just have a certain ring to it? It's just one of many moment where Arnold creates a brilliant tension between beautiful language and deeply depressing ideas.