How do you know when to put an accent on a letter and where?

When it comes to accents, it helps to say the word aloud to realise where the word's emphasis is and therefore where the accent is. For example, take the word 'decisión'. This word's emphasis is on the last syllable de-ci-SIÓN, so we know to put the accent on the vowel in that syllable.A word with the emphasis in the antepenultimate syllable is a type of word called 'esdrújula', one with the emphasis in the penultimate syllable is called 'llana', and one with the emphasis in the last is called 'aguda'. These three types of word have different rules when it comes to where the accent lies.All esdrújula words have an accent on the antepenultimate syllable.All llana words have an accent if the word DOES NOT end in an 'n', an 's', or a vowel.All aguda words have an accent if the word DOES end in an 'n', an 's', or a vowel.Therefore, we know to put an accent on the last syllable of 'decisión', as it is aguda (emphasis on the last syllable) AND ends in an 'n'.