Analysing Sonnet 02 When forty winters shall besiege thy brow

When forty winters shall beseige thy brow,

And dig deep trenches in thy beauty's field,

Thy youth's proud livery, so gazed on now,

Will be a tatter'd weed, of small worth held:

Then being ask'd where all thy beauty lies,

Where all the treasure of thy lusty days,

To say, within thine own deep-sunken eyes,

Were an all-eating shame and thriftless praise.

How much more praise deserved thy beauty's use,

If thou couldst answer 'This fair child of mine

Shall sum my count and make my old excuse,'

Proving his beauty by succession thine!

This were to be new made when thou art old,

And see thy blood warm when thou feel'st  it cold.

Famous Shakespearean sonnet, or short poem, entitled William Shakespeare Sonnet 02 When forty winters shall besiege thy brow.

In this sonnet,the words in bold and underlined aren't used anymore, they are called archaic. These words have changed, below are the actual forms.

In Northern English and Scots English, thou is still used as the second person singular pronoun.

Thou is the nominative, while the objective form is thee and the possessive is thy or thine.

The verbs that follow thou normally end in -st or est, as you can see in this sonnet.

Thy having been replaced by the possessive adjective Your.

Thine having been replaced by the possessive adjectives My, Mine.

Thou having been replaced by the pronoun You.
Art having been replaced by the auxiliary verb Are.

Couldst  Archaic the form of Could used with the pronoun thou or its relative form.