When do two vowels in Spanish form a diphthong?

Question

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Rules applied to the separation of syllables

Diphthongs (or triphthongs) are sets of two or three vowels that are pronounced as a single syllable as opposed to being separated into two. How do you determine which vowel pairs form diphthongs and which don't? Is there a rule, or do the individual pairs need to be memorized?

Answer 1

There is an orthographic rule:

1. a, e, o are strong vowels, i, u are weak vowels. y is like i.
2. There can be only one strong vowel in a syllable, they never combine into a diphthong. ca-os, le-ón, le-er
3. When a strong and a weak vowel are next to each other (or separated by h) they form a diphthong. Eu-ro-pa, hia-to, rei-na
4. There are cases when a strong vowel is next to two weak vowels. In that case, they also form a diphthong. Pa-ra-guay, U-ru-guay
5. Finally, iu and ui are also diphthongs. The second of the pair is the stronger, ie viuda where the u is predominant, Luis and Suiza the i is stronger.
6. But when a weak vowel is in its own syllable, and the stress of the word falls on it, it gets an acute accent and the diphthong is broken. ca-í-do, sa-ú-co, rí-e

But these are orthographic rules. They consider that a diphthong is any combination of letters that follow that rules, regardless of actual pronunciation. For example, people say cli-en-te but dien-te, but you wouldn't know that just by reading the words. Before, you could put an accent on the stressed syllable of some words, to show the lack of diphthong: pié (I tweeted) vs. pie (foot), enduído vs. ruido, but the new rules don't allow that.

So, to wrap it up: follow the orthographic rules and you'll know when there is a diphthong, but there'll be some false positives.

Answer 2

Once I posted an answer about calculating syllables in Spanish, the question was "Rules applied to the separation of syllables".

A guide stated that:

A diphthong is a single syllables having two vowels. It must be an unstressed closed vowel (i, u) and an open vowel (a, e, o), or two closed vowels. The possible combinations are ai, ei, oi, au, eu, ou, ia, ie, io, ua, ue, uo, iu and ui. There is always more stress on the open vowel, or if there are two closed ones, the second one. Diphthongs with two closed vowels are often pronounced separately, but orthographically they are considered as one syllable. A h between vowels doesn't make any difference.

^{Emphasis mine.}