The poem opens with a line that will become its refrain. From this one line, the reader learns three important facts:
- It's night-time;
- The speaker is a poet;
- And something emotionally traumatic has happened.
The key word here is "can," indicating that the speaker's experiences have granted him this ability to write the saddest poetry around. That doesn't necessarily mean that the speaker actually will, or even should, write "the saddest lines"! It's simply possible, and the end-stop here creates an echoing emptiness, a sense of detachment as though the speaker is merely stating the facts.
Lines 2 and 3 then directly respond to this opening, showing an "example" of one such very sad line. The fact that the speaker presents this example in quotation marks emphasizes that it's just that: an example, not necessarily the speaker actually writing "the saddest lines."
As for that line itself: on its face, a "starry" night is nothing unusual, but it pre-empts how the speaker finds painful reminders of lost love all around. Stars, so often the glinting backdrop to young lovers, in this image seem cold and alienated. They're blue, and line 3 personifies them as "shiver[ing]" in the distance. This perceived distance relates to the schism between the speaker and his ex-lover.
And with that, the speaker closes the quotation marks, leaving it to the reader to decide whether the rest of the poem follows this example or not. Given that line 4 similarly focuses on the sky and also uses personification, it seems fair to interpret the rest of the poem as more of the "saddest lines" mentioned in line 1. But it's worth noting how that "can" in the first line introduces a note of doubt and hesitancy to the whole poetic process.