Some observations about poetry forms after reading too much of it:
The sonnet has fourteen lines. Everyone knows that. What fewer people notice is how those fourteen lines create a kind of argument structure, a turn around line eight or nine where the poem pivots and says something it couldn't have said at the beginning. Shakespeare used it. Petrarch used it. It works because fourteen lines is just long enough to set up an expectation and then betray it.
Free verse isn't formless. It just hides its form. Every good free verse poem has a logic governing its line breaks, its white space, its rhythm. When people say free verse is "easier" than formal poetry, they're confusing the absence of visible rules with the absence of any rules at all.
Walt Whitman self-published Leaves of Grass in 1855, set the type himself, and then wrote anonymous reviews of his own book. He invented American free verse by writing lines so long and sprawling they broke every convention of Victorian poetry. His catalogs of ferry riders and grass blades and the bodies of Civil War soldiers created a new form by pretending form didn't matter. But read the poems closely and you'll find a very deliberate music, a rhythm built on repetition and parallel structure that's doing as much work as any metrical scheme.
The villanelle only has two rhymes. Two. The entire poem rotates between them, with two refrains that keep coming back like something you can't stop thinking about. That's why it works so well for obsession and grief. Dylan Thomas chose it for "Do not go gentle into that good night" because the form itself is a refusal to let go.
Haiku in English is almost never actual haiku. The 5-7-5 syllable count is a rough approximation of the Japanese form, which is based on sound units called on that don't map neatly onto English syllables. Most serious English-language haiku poets abandoned strict syllable counting decades ago. The form is really about a single moment of perception, a season word, and a cut between two images. The counting was always beside the point.
I'm genuinely unsure whether learning formal poetry makes you better at free verse, or whether it just makes you more self-conscious about it. I've heard convincing arguments both ways and I keep going back and forth.
Sylvia Plath wrote "Daddy" and "Lady Lazarus" in a fury during October 1962, sometimes producing two or three poems in a single day. What's easy to miss about those late poems is that even at their most raw, even when they feel like they could detonate on the page, she was using formal constraints. Terza rima echoes, villanelle patterns, precise stanzaic structures. The rage had a skeleton underneath it. The form was what let the feeling go that far without collapsing.
The prose poem is the form that makes everyone angry. Poets say it isn't poetry. Prose writers say it isn't prose. It sits in the gap between the two and refuses to explain itself, which might be the most poetic thing about it.
Most types of poetry share one trait: the line break means something. In prose, you hit the margin and the text wraps. In a poem, where you end the line is a choice, and that choice changes the meaning. A sentence broken after "I can't" reads differently than the same sentence broken after "I can't stop." Every line ending is a tiny cliff.