That's kinda like how in Spanish vowels can be either weak or strong, with a/e/o being strong and i/u being weak. When a word contains consecutive strong vowels, they're pronounced as separate syllables, such as leal pronounced LEH-al. This is known as "hiato" which there might be word for in English of which I'm unaware. But consecutive weak vowels, or a combination of a weak vowel and a strong vowel, are pronounced as a single syllable. For example, violín is pronounced veeuh-LEEN. That combination is known as "diptongo." There's an exception though, if a word's stress is on a weak vowel it becomes a strong vowel, like in río which becomes REE-uh.