Lines 5-8 Summary

Lines 5-6

Soft is the Strain when Zephyr gently blows,
And the smooth Stream in smoother Numbers flows;

• The speaker told us sound and sense needed to go together – and he wasn't kidding.
• Do you hear the way that "Soft is the Strain" is a soft-sounding phrase, but when you get to the word "Strain" the long sounding "a" isn't as easy to say? It takes longer to say "Strain" than "Soft," but even "Strain," though you strain to get to get it out, doesn't have a harsh sound to it.
• The second part of the line is just as sound sensitive. Listen to all the short "e" sounds: "when," "Zephyr" (which means a light breeze), "gently."
• The word "blows" seems almost out of place…but not quite. Why is that? There's a faint memory in our minds of a recent long "o" sound. Oh, right…in the previous line in the word "Eccho." He's done it again. The "o" sound in "blows" is an echo of the word "Eccho."
• Say line 6 as fast as you can. Hard, isn't it?
• That's because of all of those S's attached to one or two other consonants (like "m," "th," and "b"). The long "oo" and "o" sounds also kind of make you feel like you're wading through melted chocolate. It takes a little while to get our mouth around all those, well, smooth sounds.
• But wait a second. What does the speaker mean by "Numbers" here? Weren't we just talking about a nice breeze in the air? Back to that funny word prosody. "Numbers" refers to the number of syllables in a line of poetry – and all those rules about how to not give offense when writing a poem fit for society.
• The word "Numbers" in this line filled with the image of a soft breeze and smooth stream suddenly reminds us that he's not really talking about a breeze and a stream; he's talking about the way a poem works.
• The smoothness of a line of poetry is created by the way it uses its "Numbers," or meter. The basic meter for English verse is iambic pentameter, which is a technical way to say that it's written in five pairs of unstressed and stressed syllables. 
• Try marking which syllables are stressed and which ones are not in this line. (This fun hobby is called "scanning.") You'll see that the second part of the line is in iambic pentameter. If you exaggerate the sounds, the line scans like this: and the SMOOTH STREAM in SMOOther NUMbers FLOWS.
• The second part of this line enacts the meaning: "in smoother Numbers flows" is the part of the line that is in iambic meter, which is a meter that's considered smoother and more flowing than other meters.

Lines 7-8

But when loud Surges lash the sounding Shore,
The hoarse, rough Verse shou'd like the Torrent roar.

The last couplet (lines 5 and 6) was about a soft breeze and sounded smooth to match it. Now the speaker turns to the opposite extreme: loud waves beating the shore. He says that if you're writing about loud waves in a poem then the verse should be just as rough as the image.

• Have you noticed that there hasn't been a comma in the middle of a poetic line since the first line? Here, inserting a comma makes the line rough like the torrent.
• (BTW, a "torrent" is fast-moving water. This contrasts with the "smooth stream" imagery from the previous couplet.)
• What else causes the line to sound like its meaning? Instead of being smooth iambic pentameter, we get several stressed words in a row: The HOARSE, ROUGH VERSE. Yep, that sounds kind of rocky.
• In these lines, we get bits of alliteration (that's when a sound is repeated): "l," "s," and "sh" sounds. Check it out: loud lashes surges, sounding shore.
• Unlike the couplet about the soft, smooth lines, here we get louder sounds that mimic the ocean crashing.