Marvell loves the classics, and his choice of form and meter for "The Garden" is no exception. He uses a meter called iambic tetrameter, which means he's got four iambs per line. What's an iamb? It's a pair of syllables in which the first syllable is unstressed, the second one stressed: daDUM. ("Allow" is an iamb, for example.) Tetra- means four, so we know that iambic tetrameter means four iambs to a line, making each one sound like daDUM daDUM daDUM daDUM. The rhyme scheme is also pretty straightforward. Marvell uses rhyming couplets, so each of the poem's nine octaves, or eight-line stanzas, contains the rhyme AABBCCDD, where each letter represents an end rhyme sound.
This is a fairly typical rhyme scheme, but Marvell executes it here with something that's eerily close to perfection. Take a look at this:
When we have run our passions' heat
Love hither makes his best retreat.
The gods, that mortal beauty chase,
Still in a tree did end their race:
Apollo hunted Daphne so,
Only that she might laurel grow;
And Pan did after Syrinx speed,
Not as a nymph but for a reed (25-32)
Now that is some poetry. See how all the lines are perfectly 8 beats, with no extra syllables clouding up Marvell's nice, crisp meter? His end-rhymes are perfect and concise and even his syntax, or the way he grammatically structures his lines, is parallel. Folks, if Marvell were still alive we'd owe him a standing ovation for this.
Marvell wasn't just trying to flex his poetic muscles though; he may have had other reasons for adhering to the metrical pattern so strictly. English gardens in the seventeenth century, it turns out, were often painstakingly designed and placed a huge emphasis on structure, symmetry, and formality—things that some scholars believe Marvell was trying to recreate with his incredibly symmetrical, structured use of meter and form in "The Garden."