"To Marguerite" by Matthew Arnold is comprised of four stanzas of six iambic tetrameter lines each, rhymed ABABCC.
Stanza 1: This stanza introduces the notion of humans being isolated just as islands are separated by the ocean.
Stanza 2: Despite islands and the people therein being isolated from each other, the sounds of the nightingales (and thus works of art, poems, correspondence) can be heard on both sides of the water making a connection between separated people.
Stanza 3: Awareness of the people from whom one is separated results in longing and despair.
Stanza 4: Why are the lovers of the poem condemned both to have discovered and fallen in love with each other and yet forced to live apart? The poem answers the question by blaming God for separating both people and islands:
A God, a God their severance ruled;
And bade betwixt their shores to be
The unplumb'd, salt, estranging sea.