Metrical Observations

Why Shakespeare Could Never Have Been French
22nd March 2021 | Tom Scott
https://www.youtube.com/watch?v=dUnGvH8fUUc

My comment:

I think you have a good, but historically superficial grasp of French prosody.

In Alexandrine verses, yes, the prosodic stress predominates so much that you do get half lines stressed at syllable six and no other rule applies hard and fast. However, lexical accent exists: "bon requin anodin" all have stress on the final syllable (and add up to six syllables, last of which is stressed), but "bonnes requines anodines" (supposing requin has a feminine form, which I think is false) all have stress on second to last syllable, and add up to nine syllables, with stress on the eighth, and so could not fit into an Alexandrine. Unlike modern pronunciation where they would be pronounced "bonn' requin' anodin' ".

There are two main ways of filling out the six syllables of a half line, namely iambic and anapaestic : six syllables is hard and fast, but you can either put lexical stresses on syllables 2, 4 and 6 or on syllables 3 and 6, all other versions being variants of these. This means that some writers of Alexandrines do in fact alternate quatrains, each having two rhymes masculine, two feminine, with one quatrain iambic (calmer) and one quatrain anapaestic (more lively). This type of placing of the "theoretical" word stresses would have no effect if the word stresses within the phrase didn't exist, at least as pronounced in scanned verses.

Plus sometimes, the natural phrasing would not recognise the two halflines without the word stress on the sixth syllable of the first one.

Now, you said "iambic pentameter" could not exist in French. It so happens, Chaucer introducing it had a model in Dante's meter, "endecasillabi" (eleven-syllabers), which ends in unaccented syllable, and so equals an iambic pentameter with feminine ending. In French you tend more to have masculine endings, and so speak of "décasyllabes". While both Italian and French have less prominent lexical stress than English, the meter works, because you divide it into half lines : 6 + 4(5) or 4 + 6(7). It sounds like Alexandrines, except either first or last halfline is two syllables shorter.

Shelley in fact did not observe strict iambic sequence in his pentameters, but you can more or less find any iamb in Prometheus Unbound (which I tried to read and failed to persist) exchanged for a troché, like:

x/ /xx/ /xx/
or
/xx/x/ /xx/

This makes Shelley's pentameter very like the original endecasillabi.