Longfellow begins like a bookkeeper. "When I compare / What I have lost with what I have gained": the speaker opens the accounts, sets one column against the other, and reaches a verdict before the first stanza ends. "Little room do I find for pride." The phrasing is odd and rewarding. Pride is treated as something that needs space, and the balance sheet leaves it none. The poem is about how we judge our own lives, and about the suspicion that the judgement is premature. Its central idea comes in the last stanza: the tally of success and failure cannot be trusted, because "Defeat may be victory in disguise."
The second stanza moves from the cool language of accounting to something closer to confession. "How many days have been idly spent" keeps the money vocabulary alive in the verb "spent," but the loss is now time, which can't be earned back. Then comes the poem's one extended image: "How like an arrow the good intent / Has fallen short or been turned aside." An arrow suits an intention because both are aimed. Longfellow names two separate ways of failing. The arrow may simply lack the strength to arrive, or it may be knocked off course on the way. Neither failure suggests that the aim was bad. The word "good" quietly protects the speaker's motives even while the results are admitted to be poor.
Then the poem turns on a single "But." The first two stanzas are in the first person, and the third widens to a question anyone might ask: "who shall dare / To measure loss and gain in this wise?" The old-fashioned "in this wise," meaning "in this way," gives the question a slightly proverbial weight. It also hints at the pun inside it, since to measure unwisely is to be unwise. The speaker doesn't deny the earlier failures. He questions the measuring. The two closing lines are cautious in their grammar, since defeat "*may* be" victory and not "is," but they end on the image of a tide. A tide has no opinion about success, and at its "lowest ebb" it has already begun to come back in.
The form carries this argument quietly. Each stanza opens with a short, two-beat line, and these openings rhyme with one another: "compare," "aware," "dare." The longer lines that follow hurry along with many unstressed syllables, as if ticking items off a list, and "gained" and "attained" are matched in stanza one, as "spent" and "intent" are in stanza two. The last line of each stanza rhymes across the whole poem: "pride," "aside," "tide." Reading down that sequence, a reader can feel the poem's mood change, from pride refused, to good intention deflected, to the water turning. The rhyme of "wise" with "disguise" in the third stanza also fits, because wisdom here means seeing through appearances. The final line, "The lowest ebb is the turn of the tide," settles into a steady rocking rhythm, like a wave, and its short, even words make it feel like a saying that has always been true.
What makes the poem worth reading is its restraint. Longfellow was one of the most widely read American poets of his century, and a plain, memorable manner suits this twelve-line piece. It doesn't pretend that failure is painless, and it doesn't hand out easy comfort. It stays honest about wasted days and falling arrows, then argues that the person doing the counting is badly placed to know the final total. The ledger is never closed, and the poem leaves its speaker, and its reader, with a modest and believable kind of hope.