Let me not to the marriage of true minds
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove:

O no! it is an ever-fixed mark
That looks on tempests and is never shaken;
It is the star to every wandering bark,
Whose worth's unknown, although his height be taken.

Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come:
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom.

If this be error and upon me proved,
I never writ, nor no man ever loved

In the sonnet "Let me not to the marriage" Shakespeare gives a definitions of love, explaining what the real nature of love is.

From the analisys of the sonnet we can affirm that the function of the three quatrains is to give an exact definition of love: real love is constant and strong and it will not change. Thrue love is an "ever fidixed mark". Love is naturally unshakeable throughout time. In the last couplet the poet affirms that he cannot be mistaken; he must be right, no error is possible.

Shakespeare in the first quatrains defines love as an unchangeable feeling: "Love is not love, which alters when it alteration finds".

In the second quatrain love is once more defined as uncxhanging; Shakespeare continues to define love, but in a more direct way, it is a "ever fixed mark".

The poet uses compounds to be more direct. He uses figurative speach. Shakespeare wants to focus on time which goes by. Real love never alterates, and it resists to the edge of doom.

The rhymes in the first and in the third quatrain underline what thrue love is not, while the second quatrain is an affarmative quatrain, it stands by itself and the poet gives it a different sound.