sonnetXVIII

Shall I compare thee to a summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date:
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimmed,
And every fair from fair sometime declines,
By chance, or nature's changing course untrimmed:
But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow'st,
Nor shall death brag thou wander'st in his shade,
When in eternal lines to time thou grow'st,
   So long as men can breathe, or eyes can see,
   So long lives this, and this gives life to thee.

This is one of the most famous of all the sonnets, justifiably so. But it would be a mistake to take it entirely in isolation, for it links in with so many of the other sonnets through the themes of the descriptive power of verse; the ability of the poet to depict the fair youth adequately, or not; and the immortality conveyed through being hymned in these 'eternal lines'. It is noticeable that here the poet is full of confidence that his verse will live as long as there are people drawing breath upon the earth, whereas later he apologises for his poor wit and his humble lines which are inadequate to encompass all the youth's excellence. Now, perhaps in the early days of his love, there is no such self-doubt and the eternal summer of the youth is preserved forever in the poet's lines. The poem also works at a rather curious level of achieving its objective through dispraise. The summer's day is found to be lacking in so many respects (too short, too hot, too rough, sometimes too dingy), but curiously enough one is left with the abiding impression that 'the lovely boy' is in fact like a summer's day at its best, fair, warm, sunny, temperate, one of the darling buds of May, and that all his beauty has been wonderfully highlighted by the comparison.

The 1609 Quarto Version

SHall I compare thee to a Summers day?
Thou art more louely and more temperate:
Rough windes do ſhake the darling buds of Maie,
And Sommers leaſe hath all too ſhorte a date:
Sometime too hot the eye of heauen ſhines,
And often is his gold complexion dimm'd,
And euery faire from faire ſome-time declines,
By chance,or natures changing courſe vntrim'd:
But thy eternall Sommer ſhall not fade,
Nor looſe poſſeſſion of that faire thou ow'ſt,
Nor ſhall death brag thou wandr'ſt in his ſhade,
When in eternall lines to time thou grow'ſt,
   So long as men can breathe or eyes can ſee,
   So long liues this,and this giues life to thee,

Commentary

1. Shall I compare thee to a summer's day?
This is taken usually to mean 'What if I were to compare thee etc?' The stock comparisons of the loved one to all the beauteous things in nature hover in the background throughout. One also remembers Wordsworth's lines:
We'll talk of sunshine and of song,
And summer days when we were young,
Sweet childish days which were as long
As twenty days are now.
Such reminiscences are indeed anachronistic, but with the recurrence of words such as 'summer', 'days', 'song', 'sweet', it is not difficult to see the permeating influence of the Sonnets on Wordsworth's verse.
2. Thou art more lovely and more temperate:
The youth's beauty is more perfect than the beauty of a summer day. more temperate - more gentle, more restrained, whereas the summer's day might have violent excesses in store, such as are about to be described.
3. Rough winds do shake the darling buds of May,
May was a summer month in Shakespeare's time, because the calendar in use lagged behind the true sidereal calendar by at least a fortnight.
darling buds of May - the beautiful, much loved buds of the early summer; favourite flowers.
4. And summer's lease hath all too short a date:

Legal terminology. The summer holds a lease on part of the year, but the lease is too short, and has an early termination (date).

5. Sometime too hot the eye of heaven shines,
Sometime = on occasion, sometimes;
the eye of heaven = the sun.
6. And often is his gold complexion dimmed,
his gold complexion = his (the sun's) golden face. It would be dimmed by clouds and on overcast days generally.
7. And every fair from fair sometime declines,
All beautiful things (every fair) occasionally become inferior in comparison with their essential previous state of beauty (from fair). They all decline from perfection.
8. By chance, or nature's changing course untrimmed:
By chance accidents, or by the fluctuating tides of nature, which are not subject to control, nature's changing course untrimmed.
untrimmed - this can refer to the ballast (trimming) on a ship which keeps it stable; or to a lack of ornament and decoration. The greater difficulty however is to decide which noun this adjectival participle should modify. Does it refer to nature, or chance, or every fair in the line above, or to the effect of nature's changing course? KDJ adds a comma after course, which probably has the effect of directing the word towards all possible antecedents. She points out that nature's changing course could refer to women's monthly courses, or menstruation, in which case every fair in the previous line would refer to every fair woman, with the implication that the youth is free of this cyclical curse, and is therefore more perfect.
9. But thy eternal summer shall not fade,
Referring forwards to the eternity promised by the ever living poet in the next few lines, through his verse.
10. Nor lose possession of that fair thou ow'st,
Nor shall it (your eternal summer) lose its hold on that beauty which you so richly possess. ow'st = ownest, possess.
By metonymy we understand 'nor shall you lose any of your beauty'.
11. Nor shall death brag thou wander'st in his shade,
Several half echoes here. The biblical ones are probably 'Oh death where is thy sting? Or grave thy victory?' implying that death normally boasts of his conquests over life. And Psalms 23.3.: 'Yea though I walk through the valley of the shadow of death I will fear no evil ' In classical literature the shades flitted helplessly in the underworld like gibbering ghosts. Shakespeare would have been familiar with this through Virgil's account of Aeneas' descent into the underworld in Aeneid Bk. VI.
12. When in eternal lines to time thou grow'st,
in eternal lines = in the undying lines of my verse. Perhaps with a reference to progeny, and lines of descent, but it seems that the procreation theme has already been abandoned.
to time thou grow'st - you keep pace with time, you grow as time grows.
13. So long as men can breathe, or eyes can see,
For as long as humans live and breathe upon the earth, for as long as there are seeing eyes on the eart.
14. So long lives this, and this gives life to thee.

That is how long these verses will live, celebrating you, and continually renewing your life. But one is left with a slight residual feeling that perhaps the youth's beauty will last no longer than a summer's day, despite the poet's proud boast.