Accent rules

Find the stressed syllable, then ask one question: does the word end in a vowel, n or s? Agudas, stressed on the last syllable, take the tilde when they do. Llanas, stressed on the second to last, take it when they do not. Esdrújulas skip the test and always wear one.

A weak vowel (i or u) next to a strong one normally glides into one syllable: bien, cuando, seis. When the stress falls on the weak vowel instead, the pair splits, and Spanish marks the split with an accent on the weak vowel. That is why día is dí-a and país is pa-ís. This override beats the ordinary agudas and llanas rule.

The stressed syllable stays put when a word grows, but its place from the end moves. Joven becomes jóvenes, now stressed three from the end, so the accent arrives. Canción becomes canciones, now a llana ending in s, so the accent goes. Run the test again on the new word.