Vowel nasalisation: what is the underlying form of "onde"?

Question

I know little about French, but I have a question about French phonology.

I saw that French has a rule of vowel nasalisation, which nasalises a vowel before an [n] in the same syllable. After vowel nasalisation, there is an n-deletion rule, which deletes [n] if it appears in the coda after a nasalised vowel. And finally, there is a schwa-deletion rule, which deletes foot-final [ə], so that the feminine forms do not have a schwa on the surface.

For example, the musculine "plein"[plɛ̃] and the feminine "pleine" [plɛn]. Their underlying forms are /plɛn/ and /plɛn-ə/.

However, it is said that there are also many words in French like "onde" [ɔ̃də], which only have a nasalised vowel, instead of an oral vowel following an [n]. Its underlying form is not /ɔndə/, like the "plein" does.

Then what is the underlying form of [ɔ̃də]?

Answer 1

The particular linguistic hypothesis to which you refer posits that nasal vowels are the result of an underlying /VN/ (where V is any vowel and N any nasal consonant, not just /n/) in the same syllable.

So plein in this analysis is underlyingly //plɛn// (one syllable) while pleine is //plɛ.nə// (two syllables). The nasalition process then affects the syllable //plɛn// but not //plɛ//, yielding /plɛ̃/ and /plɛ(ː)n(ə)/.

Applying this analysis to onde then, we'd start with an underlying //ɔn.də//, yielding //ɔ̃.də// after the nasaltion rule, then /ɔ̃d(ə)/ once the optional schwa deletion rule is applied.

I'll note that this analysis, like all hypotheses who try to posit that a diachronic sound change has become a synchronic phonological operation, is really vulnerable to exceptions to the sound change and to later changes or borrowings. Even ignoring newer loanwords, one issue are words such as hymne /imn/ (that was /i.nə/ at the time nasalisation happened, with the /m/ inserted later by reading pronunciation), that this hypothesis would predict should be pronounced /ɛ̃n/ (from an underlying //im.nə//). Another are pairs such as ennui /ɑ̃.nɥi ~ ɑ̃.nwi/ and innoui /in.nu.i/ or /i.nu.i). If you posit that the underlying form of ennui is /an.nʏ̯i/ (as you'd have to do to explain its nasal vowel by this hypothesis), then the absence of nasalisation of the geminate form of innoui becomes puzzling.