"On My First Son" is written in the most popular meter for English poetry: iambic pentameter. In each line of the poem, there are five (which is where the "pent-" comes from) groups, each called a foot. In each foot, there is an unstressed syllable followed by a stressed syllable. That pairing is called an iamb.
As an example, take a look at line 12: "As what he loves may ne-ver like too much." Better yet, read it slowly out loud to yourself. Hear that regular back and forth rhythm? That's the sound of some sweet, sweet iambic pentameter.
Not every line breaks down perfectly, however (what would be the fun in that?). Take a look at line 3: "Se-ven years thou wert lent to me, and I thee pay." First, notice that the first foot (or group) begins with a stressed syllable followed by an unstressed syllable. This pattern (stressed, unstressed) is called a trochee, and it is perfectly fine to substitute one for an iamb. You won't get pulled over by the poetry police or anything. (Psst. There are some other trochees in the poem, too. Can you find them?)
Second, see how this line has six groups (feet) instead of five? A line that has six feet, or groups, of iambs is called a hexameter line or, more commonly, an Alexandrine. As it turns out, line 3 is the only hexameter line in the poem. So why does Jonson lengthen only this particular line?
One possibility might have something to do with the fact that this is the only line in the poem where the speaker mentions his loss directly. Sure, the whole poem is about how the son's death makes him feel, but line 3 really directly states how the speaker has to give his son back (to God or Heaven) with the phrase "I thee pay." Rhythmically we get thrown off here. We were just cruising along, all happy in our iambic pentameter and then clunk! This Alexandrine shows up.
It's almost as if the speaker stumbles here when he has to deal directly with losing his son. This line in particular, where we learn how old the son was when he died, is the most detailed biographically. It seems like the speaker is having a hard time getting all the words out, though, in a way that flows. Of course, that makes sense. Death is really hard to talk about, so maybe we'd be wrong to expect a nice, smooth rhythm throughout this poem.
Ben Jonson's poem also rhymes, and since it rhymes it must have...yep, you guessed it: a rhyme scheme. This is the rhyme scheme for the poem: AABBCCDDEEFF. The last word of an A line rhymes with the last word of the other A line. B rhymes with B, C with C, and so on.
Notice how there are six rhyming groups? Each one of these groups is called a couplet, which means two lines in succession that rhyme with each other. They are called couplets because they are like little couples. Cute, huh?
More than just being adorable, though, these couplets also seem appropriate for this poem, which deals with a pair. The father-son bond is in a way recreated by these lines. As the father and son are joined by love, these lines are joined in rhyme. We think that's pretty sweet, don't you?
The other thing you need to know about Jonson's poem is that it is an elegy. An elegy is a poem about a person who has died (it can be about a group of people as well, but they should be dead for the poem to be a real elegy). Elegies usually consist of three basic parts: a mourning of the dead, a celebration or praise of the dead person, and finally some form of consolation. (For giggles—or sniffles—see if you can locate these three stages in "On My First Son.")
Oh, and by the way, remember that hexameter line we told you about earlier? Well, in the ancient elegies (written by the Greeks and Romans a few thousand years ago), the lines alternated between dactylic hexameter, followed by pentameter. So, Jonson's hexameter in line 3 isn't totally random, is it? He's just following an ancient example.