Let’s start with the diphthong, this is the result of the combination of two vowels in the same syllable, it occurs when these vowels are one weak or closed, one strong or open, or vice versa, it also occurs if the combination is composed by two weak vowels.

Weak, closed, open, strong? I know only  A, E, I, O and U.

Weak: I, U.

Strong: A, E, O.

In these combinations, both vowels form the same syllable. That is to say, they are dipthongs.

Example:  Closed vowel with weak and vice versa: Auto, Guapo (au-ua).

                  Closed with strong and vice versa: Diamante, Aire (ia -ai).

                  Strong with closed and vice versa: Neumático, Nuevo (eu-ue).

                  Closed with strong and vice versa: Nieto, Pleito (ie-ei).

                  Weak with weak: Viuda, Buitre (iu-ui).

There is an exception to these combinations, which occurs when we put together two strong vowels or is conditioned by the stress of a weak vowel. That is, the weak vowel will be tonic and the strong vowel will be unstressed. The result is a hiatus.

This phenomenon of prosody is represented graphically with the accentuation on the weak vowel, i.e. the weak vowel will always carry the accent mark, to show the rupture of the diphthong. 

Examples: Two equal open vowels, Leer, S, Alcohol, Azahar. Since the H has graphic

                  representation, but not phonetic, although the H is intercalated, this 

                  combination is considered a hiatus. 

                  Two distinct open vowels: Marear, loable, cgulo, pn, cacao, Koala. 

                  Closed vowel with strong vowel and vice-versa: Río, herna, dúo, búho, lúa


Note: The combinations of strong vowels must be submitted to the Spanish accentuation rules, whether they are agudas, graves or esdrújulas