How is sonnet 55 and sonnet 43 alike?
They are both sonnets, and Shakespearean sonnets. They are both addressed to someone described as "you". They are both, to a certain extent, praises of the person to whom they are addressed. Sonnet 55 uses the phrase "But you shall shine more bright" which echoes the primary image of Sonnet 43, as seen in the line "thy much clearer light".
Although both sonnets deal with a contrast, they are different contrasts. Sonnet 43, with its constant imagery of light and shadow, contrasts what we see in dreams with what we see when we are awake. Sonnet 55 contrasts the immortality of the subject of literature with the ephemeral nature even of brick-and-mortar monuments. In Sonnet 43, Shakespeare is saying that he has clearer vision in dreams because of the presence of the person to whom…
There is only one couplet in Shakespeare's Sonnet 55. As in most others of his sonnets, it provides the concluding two lines of the poem and it summarises the theme of the whole sonnet (in this case, that the addressee will be immortalised by the poem). The couplet may be rendered as follows in modern English: So, till you, on the day of judgement, rise You'll live in this, and dwell in lovers' eyes.
We cannot say with certainty. However, there is considerable evidence to suggest that all the poems in Sonnets 1-126 were addressed to Shakespeare's patron, Henry Wriothesley. In Sonnet 55 he appears to continue his flattery of the Earl through a recurring theme of immortality through verse (picked up here from the closing lines of the preceding sonnet). Read more in the link below.