The speaker begins by saying that he grieved when "lilacs last in the dooryard bloom'd," meaning that he went through a period of mourning during springtime (which is when lilacs bloom). He also sets the scene by describing a bright "star" on the western horizon (most likely Venus, the brightest thing in the western sky during spring). The speaker also adds that he will "mourn" each time spring returns and brings back these images—images that evoke thoughts of a lost loved one ("him I love").
Although the speaker doesn’t directly name of this loved one, the context of the poem helps make sense of these opening lines. The poem was published in 1865, the same year President Abraham Lincoln was assassinated. Written in the wake of this national tragedy, the poem describes the speaker's grief and sense of loss surrounding the president's death. Instead of stating the president's name, the speaker expresses his grief through imagery and metaphor.
To that end, the fact that the speaker mentions lilacs is significant because they bloom in the spring, thus representing the first signs of new life after winter. This hints at a kind of metaphorical winter, as if Lincoln’s assassination plunged the nation into a state of mourning that was as cold and unforgiving as a harsh winter. The image of the lilacs therefore suggests that new life is possible even after loss, even if this new life also brings with it a renewed sense of grief.
The star in the western sky is also symbolic. In the Bible, the star of Bethlehem signaled the birth of Christ. But in the poem, the speaker describes the star as falling from the sky, thus symbolizing Lincoln's premature death while still subtly presenting him as a Christ-like figure, someone who may redeem the country as a whole.
The speaker reaffirms this biblical connection by saying that spring will always bring back this “trinity” of the lilac, the star, and the thoughts of Lincoln. The “trinity” alludes to the biblical trinity of the Father, Son, and Holy Ghost. But the “trinity” in this poem consists of ordinary things in the natural world—the lilac bush, the star in the sky, and even memories of someone who has died. The speaker suggests that these things too are sacred, and that in losing Lincoln, the nation has lost a spiritual leader.
Several poetic devices in these lines reinforce their meaning. First, the speaker repeats the word “mourn,” in the past tense (“mourn’d”) and then in the future tense. This polyptoton conveys the sense that the speaker’s grief and mourning is ongoing.
At the same time, the repetition of the phrase “ever-returning spring” suggests that spring too is eternal and continuously returning. Taken together, this repetition suggests that mourning and death are a constant for the speaker, but so too is the reality of springtime and new life.
The internal rhyme of “spring” and “bring” affirms this sense, showing that spring will always bring a renewed sense of grief for the speaker, but will also offer other things—such as the scent of lilacs. Finally, the alliterative /l/ sounds in “lilacs" and “last” simply adds a musical effect to the poem's opening line, showcasing the speaker's lyrical and nostalgic tone.