Sonnet 18 is one of Shakespeare's most well-known and recognized pieces of poetry.
The first quatrain is a comparison of a young man to a summer's day-and the outcome is "thou art more lovely and more temperate". The young man is perfection, and outdoes even nature. The poet is unable to give adequate crdeit to all the young man is.
The second quatrain describes the conditions that will affect such perfection. Time is fleeting, and perfection is not permanent. It is hard to remain perfect where mortals are concerned. The summer is only one season in the year, and this refers to the inevitable aging that will occur.
The last quatrain suggests that through this poem, the young man will achieve immortality. The poet has written lines that will keep the young man alive in the minds of the reader. "So long lives this, and gives life to thee".