How we cite our quotes:
Like as the waves make towards the pebbled shore,
So do our minutes hasten to their end, (1-2)
The speaker begins the poem by calling our attention to the passage of time, and how everything that happens must… stop happening at some point or another. This idea is doubly emphasized, both by the word "end" in line 2, but also more concretely in the image of the "pebbled shore" where the waves crash and shatter at the end of their long march toward land.
Each changing place with that which goes before, In sequent toil all forwards do contend. (3-4)
Here, in the next two lines of the poem, the speaker brings in another idea: that time only flows (so to speak) in one direction. This idea becomes especially powerful in line 4, with its emphasis on the hard work ("toil") and struggle ("do contend") involved as the waves—and minutes—make their way relentlessly forward. We'd almost feel sorry for time, if it weren't messing with our youthful beauty.
Nativity, once in the main of light,
Crawls to maturity, wherewith being crown'd,
Crookèd eclipses 'gainst his glory fight, (5-7)
In the middle of Line 6, everything turns on a dime—or rather, turns on the word "wherewith." By taking us from the glory of the sun to its downfall so swiftly in the middle of a line, Shakespeare reminds us of that notion of the waves "each changing place with that which goes before" from Line 3. Things can change in the blink of an eye, Shmoopers. It's best to keep that in mind.