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Source: https://writers.stackexchange.com/a/7896
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Source: https://writers.stackexchange.com/a/7896
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

I should warn that I'm actually not really qualified to answer this question since I neither know too much about a poetry nor am I even a native English speaker. However I'm answering anyway because I've made an observation which might adequately explain why the first version sounds fine and the second doesn't.

Looking at your first version,

> In poetry, it's true, it can be changed,

I notice there to be a second rhythm on top of the "stressed-unstressed" pattern: The stressed syllables alternate between "strongly stressed" and "lightly stressed". Let me make it explicit by making the strongly stressed syllables **bold** and the slightly stressed ones _italics_:

"In **po** -e-_try_ it's **true** , it _can_ be **changed**"

As you see, "secondary stressed" syllable of "poetry" ends up in a lightly stressed slot.

Now for your second version:

> It's true, in poetry, it can be changed,

Adding the very same pattern here results in

"It's **true** , in _po_-e- **try** , it _can_ be **changed**"

As you see, now the "secondary stressed" syllable goes into a strongly stressed slot. And I think that is what makes it sound wrong.

Addition:

Looking at the Shakespeare lines quoted by Standback, they also seem to follow an alternating pattern of strongly/lightly stressed syllables, except they start with the lightly stressed one:

> Shall _I_ com- **pare** thee _to_ a **sum** -mer's _day_?  
> Thou _art_ more **love** -ly _and_ more **tem** -per-_ate_

Note how, again, the "secondary stressed" syllable of "temperate" goes to a lightly stressed one.

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