Interesting question. Longfellow, as a Romantic poet, relies carefully on meter and rhyme schemes to present his moral-teaching poetry. "A Psalm of Life" is mainly trochaic tetrameter which means that most of its lines have four sets of two-syllable pairings (feet); this is where the word tetrameter comes in--to describe the four feet. Trochaic means that in each of those parings or feet, the words have a stress on the first syllable and then an unstressed syllable. Below I've listed the first line of the poem and have italicized the stressed portion of each foot.
"Tell me not in mournful numbers,"
If you read this out loud, its beat should sound like: Dum-da, Dum-da, Dum-da, Dum-da. I usually beat out the lines of poetry on my desk to get a feel for what its rhythm should be.
That being said, in each of Longfellow's four-line stanzas, generally the second and fourth lines are missing half a foot or one beat--so instead of having four feet total, those lines have three and a half. This could be Longfellow's clever portrayal of how life can cut off before you think it should be finished, which would certainly illustrate the theme of "A Psalm of Life." Or, it could simply be Longfellow's demonstrating his skill with handling meter and putting his own spin on it.