This poem is about the love, first, and later the hate a man feels towards who was his beloved because she left him. It’s a very typical Romantic poem, typical of a Romantic writer like Lord Byron, who expresses his feelings of love, a typical issue of Romanticism.
With respect to the structure of the poem, When we two parted contains four stanzas of eight verses each one. There are, also, four kinds of rhyme in each stanza. In the first four verses of each stanza, odd verses have a rhyme, and even verses have another rhyme, and this method is used by the writer in the last four verses of each stanza with a rhyme for each pair of verses as I said before. The vocabulary is easy to understand for everybody who studies the English language. The first verse of the poem is also the title of the poem, which means that the writer could not or did not want to find a title for the poem (maybe the damage he felt was so strong that he was not able to find a good title for the poem, as he writes at the end of the third stanza (Long, long shall I rue thee/ Too deeply to tell).

As I said at the beginning of the commentary, firstly the poem is about the end (the man probably does not know that it is the end) of a love story between a man and a woman (the first four verses). After the whole poem is an expression of feelings of hate from the man towards the woman because she left him (To sever for years: did the man go to fight in a war, for instance?). Also, the whole poem, from the first line to the last one, is written in the first person, in plural (When we two parted), and in singular (I hear thy name spoken). It means that this poem maybe autobiographical or not, maybe the woman was a real woman or not, or simply the work is an ode to disappointed love. I would like to emphasize the contrast between two verses (the second one and the last one), seemingly equal in form but with different meanings, it depends on what the previous line said. In the first pair of verses (When we two parted/ In silence and tears) the man feels true love towards his beloved, but she, as we will see some verses later, does not, and the last verse is the definitive confirmation of what he feels is disgust and pity because of what could be and was not. The third verse of the poem may have two different meanings too: Half broken-hearted: the man can have his heart broken because he loves her completely and he would do anything for her, and can have his heart broken because she has left him and he will never see her again.

The cold is a very important metaphorical element at the beginning of the second and the third stanzas: The dew of the morning/ Sunk chill on my brow: here the cold is shown as a metaphor of the feelings that the man has towards the woman, as it happens too in A knell to mine ear/A shudder comes o’er me: dew, chill, knell and shudder are words that symbolize cold in many different ways. Here the man would not already break himself his heart for her. The verses They name thee before me and They know not I knew thee give to understand that maybe more men were in love to the woman he got for some time.

In the last stanza, the two first verses have two words that may be synonyms, but they mean a totally different thing, they are the contrast of the poem: In secret we met, here Byron wants to transmit the passion of two lovers in their first secret encounter. And In silence I grieve symbolizes that nobody can help this man to come back to smile after having been left by the woman he loved. The two following verses, That thy heart could forget/ Thy spirit deceive say what she made him: she forgot him and she deceived him.