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 dimm'd;
And every fair from fair sometime declines
By chance or nature's changing course untrimm'd:
But thy eternal Summer shall not fade,
Nor lose possession of that fair thou owest;
The word “fair” — used in Shakespeare’s time to mean “beauty” — is used as a both a noun and an adjective. The line can be translated as “from every beautiful person some of that beauty is ultimately lost. The adverb "sometime” is an archaic word meaning eventually. 
 Shakespeare’s sonnets are characterised by their compelling rhythm, as in this line with the repetition of “fair”.
Sonnet 18