Three against one

[AngelicaDeAlquezar]

Indeed: the diphthong is "destroyed" because the accent turns a "weak vowel" into a "strong vowel" and two strong vowels never make a diphthong.

Strong vowels: a - e - o
Weak vowels: i - u - (y)


Strong-strong => hiatus
- a·e·ro·pla·no
- ca·os
- pe·le·a
- le·ón -> This hiatus is not introduced by the accent; the accent is necessary because this is a two-syllable "palabra aguda" that ends in "n".
- bo·a
- a·lo·e

Weak-weak => diphthong
- diur·no
- cui·da·do
- muy

Weak-strong => diphthong
- I·ta·lia
- die·ta
- ca·na·rio
- cua·li·dad
- nue·vo
- va·cuo

Strong-weak => diphthong
- frai·le
- rei·na
- co·loi·dal
- au·men·to
- feu·dal
- hay
- rey
- doy

Accented strong - strong => hiatus
- ha·rí·a
- guí·a·me
- tí·o
- pú·a
- ce·bú·es
- bú·ho

Strong - accented strong => hiatus
- a·hí
- le·í·a -> notice the double hiatus
- o·í
- ba·úl
- re·hú·sa·te
- mo·hí·no


Weak - accented strong = hiatus
- cu·í·da·lo
- hu·í·mos


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- in·ge·nie·rí·a
- za·na·ho·ria
- a·e·ro·puer·to


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