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Could you explain to me why the combinations of two vowels are stressed differently? Are both of these combinations diphthongs?
Thank you
Edit: I found the proper explanation here http://www.learn-spanish-amigos.com/diphthongs-in-spanish.html
But now I do not understand this part there:
If the first of 2 weak vowels coming together is accented, two syllables are formed:
Fluido (Flu-i-do) (fluid)
The webpage you consulted makes a dreadful mix of phonetics (how things are pronounced) and orthography (how things are written) and is also not really using proper terms. So let's begin by that.
The so-called "strong vowels" are in fact the open or low vowel /a/ and the mid vowels: /e/ and /o/. A "strong" vowel is always syllabic and can always receive stress.
The so-called "weak vowels" are the close or high vowels: /i/ and /u/. The special thing about these high vowels is that they can become non-syllabic, i.e. semiconsonantic, pronounced [j] and [w] respectively (like the English glides written y and w). Non-syllabic vowels are by definition part of a diphthong.
Two vowels next to each other in Spanish may or may not form a diphthong. If they don't, they are said to be in hiatus.
Strong vowels next to each other are always in hiatus.¹
A strong vowel next to a weak one (in either order) can form a diphthong or not. If they don't, then the weak vowel must be marked with an accent mark (rí-o "river", o-ír "to hear"). A strong+weak vowel group in an unstressed syllable is always a diphthong.²
Now on to your question:
Two weak vowels next to each other are most often pronounced as a rising diphthong (ui = [wi], as in English we or French oui; iu = [ju], as in English you). The exceptions are few: fluido is often pronounced as three syllables, with a hiatus between u and i, and muy has a falling diphthong ([muj]). You can also find a hiatus in the verb huir [u.ir] and its derivates (depending on who's speaking), though not in fui [fwi], etc.
The combination iu is also almost always a diphthong, as in ciudad, viuda, diurno, oriundo, but it can have a short hiatus in triunfo. This might have something to do with the fact that there's a glide r in the syllable onset, in the same way that there's an l in fluido.
Notes:
¹ Strong vowels next to each other are always formally in hiatus, but that's not really the case in fast speech, where unstressed e and o often become higher (closer to i and u) and non-syllabic. This is common across word boundaries and recognized especially in poetry (synalepha).
² A strong+weak vowel group in an unstressed syllable is nearly always a diphthong. As pointed out in this question about verbs ending in -iar and -uar, there are instances of unstressed ia, ie, ua, ue in some conjugations where the vowels are pronounced in hiatus. These verbs include fiar, enviar, adecuar and actuar.
There is no rule on how to place stress in combinations of vowels. It is the other way around. Any two vowels can be stressed in any way and, depending on whether they are both in the same syllable, and whether they are strong or weak, they either form diphthongs or they don't.
In your examples the vowels are divided into syllables like this:
+ Causa = Cau-sa.
+ Veamos = Ve-a-mos
So in causa the two vowels are in the same syllable and in veamos they are in different ones.
The combination "au" is considered a diphthong, as you already found, and the combination "ae" is not, as they are both strong vowels.
As for the second question: As I said before, any vowel could be the stressed one, so the reference says that in case of two weak vowels together if the stress is on the first one then they are pronounced as two separated syllables. And if they are in separated syllables then there is no diphthong. Again my point is that it is not that the combination of syllables tells you how to pronounce the word, but the pronunciation tells you if it is diptongo o hiato or neither.
After trying to look for other examples I think the fluido example shows that perhaps the reference is wrong and it should say second instead of first.
The syllable split for fluido in the reference is fine but the stress is on the i (flu-I-do) because if the stress were on the u then the word would be considered esdrújula for having the stress on the third syllable (counting backwards) and then it would be written flúido. The main thing is that the pronunciation would put the two vowels in different syllables so there would be no diphthong there.
An example of a diphthong with ui combination is buitre (bui-tre) where both vowels are pronounced as one syllable.
I think you got confused thinking that this is a rule on how to pronounce a combination of vowels but for me diphthongs are not ways to help you pronounce words. The way to know how to pronounce a word is a combination of knowing this plus watching if the word has a "acute accent" (tilde á é í ó ú) and knowing when these tildes are used.
