We already know that all syllables must contain at least one vowel. But we also know that the same syllable could contain two or even three vowels. In these cases we speak of diphthongs and triphthongs, and when two vowels go together but not scrambled, we call it a hiatus. We will try to define these concepts with their respective examples.
Each syllable must be formed by a single voice stroke, so that in a syllable we find more than one vowel, it is necessary that one of those vowels does not involve any additional voice stroke. If that happened we would not be facing one syllable, but two. But let's take an example, which is always more obvious than the whole theory. For example, the word four. Cuatro is bisyllable, but we can see that it has three vowels. This is possible because the first two (u-a) are within the same syllable. If you look closely, despite the fact that two different vowels are present in that syllable, we use a single stroke of voice to pronounce it (cua, and not cu-a). On the contrary, if we think of the word bring, we see that the vowels a and e, even when physically together, are well separated by different voice strokes. We do not pronounce bring, but bring-er. That's the difference between what are diphthongs and what are not.
And the tripthong? Well, as you can imagine, there is a tripthong when three vowels come together in the samesyllable. It is rarer, but we also have numerous examples in our language. The rule is the same as for the diphthong: although there are three vowels, the syllable is ventilated with a single stroke of voice. This rule is unbreakable. Let's think of an example: an animal, the ox. Ox has three vowels (the Greek y acts here as Latin i, for grammatical purposes), but if you look closely at your pronunciation, you only use a stroke of voice to say it out loud (ox, not ox-ey, not ox-y).
The hiatus is easier and, in fact, we have already seen it, although we have not explained it. It occurs when two vowels together are not part of the same voice stroke, that is, even though they are together, they are part of different syllables. Earlier we gave the example of bringing (pronounced tra-er). We could also say croak (cro-ar). The word hiatus comes from the Latin hiatus, which means rupture, so hiatus is something that separates, that breaks. In this case, it breaks diphthongs.