Everything in this tiny poem turns on one blunt word, and that word is the title. A "sucker" is someone who falls for a trick, a gullible person who keeps getting taken in. It is also, in gardening, the name for a shoot that springs up from the base of a plant. Both meanings suit a poem about spring, and the poem seems to know it. Before the first line begins, the speaker has already admitted to being a fool.
The first line is cleverly unsettled. "green when the season comes" has no punctuation and no stated subject, so "green" can attach itself to different things. It can describe the world as spring arrives: new leaves, wet grass, the obvious colour of the season. But the next line, "I believe again," lets the word swing round to the speaker, because "green" also means naive and inexperienced. Read that way, the speaker turns green as the season comes, as raw and credulous as a seedling. The colour of renewal and the colour of foolishness are the same colour, and the poem does not choose between them. The small word "again" matters too. The belief is not new. It comes back every year, as reliably as the leaves.
The second stanza steps back and judges the first. "that old untruth" is the speaker looking at his own faith and calling it a lie, yet the demonstrative "that" sounds fond, like pointing at a familiar bad habit. Then comes the strange phrase "of a melancholy lark." The lark is traditionally a bird of morning and joy, so a "melancholy" one is a contradiction. My reading, which the text invites but does not confirm, is that the untruth is the old poetic habit of hearing human feeling in birdsong, of making a bird sad because we are. On that reading the speaker is a sucker not only for spring but for poetry itself, for its oldest and most worn-out consolations. He knows the lark isn't really melancholy, and he believes in it anyway.
The form is as spare as the idea. There are two couplets, no rhyme, no punctuation, and no capital letters except the pronoun "I", which stands out as the one assertive thing in the poem. The lines are short and uneven: six syllables, then five, then four, then seven. The lengthening matters. "that old untruth" is clipped and flat, like a verdict. The last line then unspools with the long, slow word "melancholy" and comes to rest on the single heavy syllable "lark." The sound reinforces this. The long *e* in "green," "season" and "believe" runs through the opening couplet like a bright, open vowel, and the soft *l* sounds of "melancholy lark" blur the ending into something wistful. The first stanza ends on "again," with nothing to close it. The second ends on a noun, a bird, and the poem stops, as if the sentence has been interrupted by birdsong.
What makes this small poem worth reading is how honestly it describes a mixed feeling. Many poems about spring simply celebrate it. This one admits that knowledge doesn't protect us. We can know perfectly well that renewal is a story we tell, that nature is not consoling us and that the lark is not sad, and still feel the old belief rise as the green returns. The speaker doesn't resist this or claim to be wiser than his own hopefulness. He names himself a sucker, lightly and without bitterness, and the title becomes less an insult than a self-portrait.
The compression is part of the point. In the manner of a haiku, the poem leaves a gap between its two images and trusts the reader to feel the connection. Between belief and untruth there is hardly any space, and the poem suggests that this is how belief usually works: we see through it and are carried along all the same.