I considered the website you suggested to me but I find it difficult to rewrite it in prose. I simply varied all the instances of “I” with “he” , that is the poet, but I don’t know how to simplify it and how to vary “you”, “your”. Please help me!!

1) In the first two quatrains the poet dwells upon the description of the passing of time and the decay of natural things whereas in the third the passing of time is linked to the theme of old age (???).
2) The sonnet’s meaning is contained, as usual in Shakespeare, in the final rhymed couplet: nothing can oppose time’s destructive power except marriage and procreation.
3)Rephrase
When he looks at the clock and notices the time ticking away, and sees the splendid day sink into hideous night; when he sees the violet wilt and curly black hair turn white with age; when he sees tall trees that once provided shade for herds now barren of leaves, and the summer’s crops tied up and hauled to the barn (I need to simplify this) as if summer itself were an old man being carried off to his grave—then he has doubts about the fate of your beauty (the fair youth), since you (?) too will have to undergo the ravages of time.
4) Sweet and beautiful creatures don’t stay that way; they die as fast as they see others grow. And there’s no defense against Time’s destructive power, except perhaps to have children—to defy Time when he takes you away.

It all sounds fine -- except I'd use "she" and "her" (etc.) instead of "you" and "your."