I hope this helps a bit
Fair is a noun here (“beautiful thing”)
Everything beautiful eventually loses its beauty.
Shakespeare relates his experience of this beauty to a sense of fairness. Implying that he is gifted in his chance just to see this person, this is further reinforced later in the poem, in the last two lines.
In Shakespeare’s day, a time without plastic surgery and knowledge of skin and teeth hygiene, it would have been unlikely to meet a woman who was in the prime of her life and a true beauty.
Answer:
[Fair] means (Beauty), ‘every fair from fair sometimes decline’ which means that
