One day I wrote her name upon the strand
-- Speaker (Line 1)

Importance: This first line is highly narrative. It establishes character, place, and time; and contains a specific action. This is unusual for a sonnet, a form which tends to be more contemplative than narrative. It sets this poem apart in style and in how it treats the sonnet form.

Vain man', said she, 'that dost in vain
-- Beloved (Line 5)

Importance: This is an unusual quote. In most sonnets, the only character one could quote is the speaker himself (and it is nearly always, at least in early modern poetry, a male speaker addressing or discussing a woman). Here, the beloved speaks for herself, critiquing or perhaps gently teasing the speaker for his pointless attempt to write on the ever-changing sand.

A mortal thing so to immortalize
-- Beloved (Line 5)

Importance: The beloved continues to speak for herself here. She argues that the speaker is engaged in a pointless exercise...