Starting with the word "When" which may serve as a conjunction or noun gives the descriptions that follow an air of inevitability; a consequence whose fruition or nature cannot be altered. The use of the symbolic number "forty"(a veiled Biblical allusion) conjures a sense of abundance, an image of a long period, a lengthy and gruelling period as endured by the Son Of Man in the desert. Indeed the Fair Youth will suffer as "forty-fold winters" will "besiege" his brow. Seemingly like a hostile army as the word besiege usually entails. The theme of time passing is pretty glaring at this stage and yet it is not just time but difficult years described as winters. They leave his face wrinkled, a wrinkling only worthily described through the alliteration in "dig deep", calling the wrinkles "trenches" strengthens the afore-mentioned idea of the winters being army like. The Fair Youth is at war with the winters thereby also mirroring his current battle with Shakespeare.
In line 3 and 4, Shakespeare then uses the metaphor of clothes depreciating. The degradation from "proud livery" to "tattered weed" and "gazed on now" to "small worth held" serves to vivify the severity of what the Fair Youth is attempting to battle. She wants to face head on one of life's inevitabilities, redefining the fabric of life string by string.She is pushing for immortality, a power well beyond conception.
The persona goes on to allude that the masses will remember, "Then being asked" and expect a store of that beauty from "lusty days" even called a treasure for sheer expression of its worth. A lesson for all this seems to be from William Shakespeare. One will be expected to account for their youthful days , days where one is most vibrant, days where one should establish an heir. Make sure to multiply in your years of fertility the poet would be saying.
There does seem to be at least a tinge of sexual innuendo in "lusty" as it may be taken as the poet referring to more sexually active days in which period it would only be fitting that one produces an heir. This interpretation would authenticate claims that the poet is indeed engaging in some sort wooing process where the subject of the poem is to agree to interactions of a sexual nature.
An "all-eating shame," an acid shame, a carcinogenic shame will fill denuded eyes. The apex of shame will be reached by the Fair Youth should she fail to heed Shakespeare's warning. The description of the shame is so harrowing that it serves as a climax for the poem. If such a gothic image cannot convince the Fair Youth to fall under the persona's wing, very little else will. Much like most of the poem, the evils that lurk in the shadows of old age are of a higher level, a level of hardly conceivable horrors because shame by nature would strike much harder as it comes from within. A sense of empathy might overcome the reader of Sonnet 2 at this stage. The Fair Youth is being cornered into submission and the poet senses that as the tone from then forward grows progressively softer. Almost just like a loving tone.
From line 9 the poem switches to a positive outlook in stark contrast to the lines before. It seems the persona has taken a more imploring stance, outlining the positives. Shakespeare tries to lure the subject with vanity, "more praise," a fool-proof human weakness, particularly shrewd of the persona to change strategy.
The last couplet is basically a story of renewal,life rising from smouldering ashes. A phoenix of some sort, a phoenix of one's making, an heir from one's loins "new made" and to see "warm blood" within one's offspring. In as much as it may not be clear whether the poet has succeeded and yet from the descriptions in the poem one would think it is convincing enough.