Till a' the seas gang dry, my dear,
And the rocks melt wi' the sun;
With a healthy dose of repetition, the speaker tells us again that he will love his "bonnie lass" until the seas "gang dry"; he also tells us he will love her until the "rocks melt wi' the sun."
In the line 10, you have to pretend the word "till" is at the beginning; the lines are saying "till a' the seas…and till the rocks."
"Till" is just a shortened form of the word until, and "wi'" is a shortened form of the word with, just in case you guys were wondering.
What does he mean by rocks melting with sun? Does he mean when the rocks melt in the sun? Or does he mean melt at the same time as the sun is melting?
Like the sea going dry, it is unlikely that rocks are going to "melt" (unless they get thrown into a volcano, or a meteor strikes the earth) so the speaker is again emphasizing the fact that he will love her forever or at least until long after their lives are over.
I will luve thee still, my dear,
While the sands o' life shall run.
Oh for crying out loud, we get it, dude. You really dig this girl.
Yet again, the speaker pledges that he will love his lass for a really long time—as long as he lives, to be exact.
That's where that "sands o' life shall run" comes in. It's an interesting phrase, don't you think? It means, "while I'm still alive." So the metaphorhere is of an hourglass, or some other device that measures time with sand.
The words, however, make us think of the "sands o' life" running out; the phrase "I will luve thee still" makes us think the speaker wants to say "I will love thee still, even after the sands o' life shall run out." He doesn't say that, but we can't help thinking it, can we?
Form-wise, things have gotten a little shakeup. We've got a new rhyme scheme on our hands, because in these final two stanzas, not only do the second and fourth lines of each stanza rhyme, but the first and third do, too. This pattern is commonly referred to as—wait for it—common meter. Check out our "Form and Meter" section for more.