How do accent marks work in Spanish?

Accent marks, or 'tildes' can be tricky in Spanish, and in some cases they are just diacritical marks, meaning they indicate that two words have different meanigs as in 'más' (more) and 'mas' (but). However, accent marks often fall in four categories depending on where the stress is in a word:palabras 'agudas': words stressed on the last syllable. They have an accent mark when they end in a vowel, n, or s. example: sofá, amor.palabras 'llanas': words stressed on the penultimate syllable: They have an accent mark when they DO NOT end in a vowel, n, or s. example: cama, bíceps. palabras 'esdrújulas': words stressed on the antepenultimate syllable: they always have an accent mark. words with a 'hiato'. 'Hiatos' are formed by a closed strong vowel (i,u) and an open weak vowel (a, o, e) and they always have an accent mark on the closed strong vowel. example: sabía, había.