Spanish vowels are either strong or weak.
The vowels a, e and o are strong.
The vowels u and i are weak.
When two strong vowels appear next to each other, they form two separate syllables.
When a vowel appears next to a weak vowel, they together form one single syllable.
The two vowels that appear together in one syllable are called diphthongs.
Let's look at some words with strong and weak vowels to see how they are divided in syllables.
If we want to prevent the weak vowel from forming a single syllable, then we put an accent mark on the weak syllable, thereby breaking the diphthong.
Putting an accent mark on the strong syllable within a diphthong does not break the diphthong.