Rules for written accents:
sótano = basement palabra esdrujula (3+ syllables) always written accent
word with 3 sylabes = so - ta - no
spoken stress falls
before the penultimate syllable, so accent is written (in general any word with 3 or more syllables stressed before the penultimate syllable has written accent)
árbol = tree palabra grave
word with 2 syllables = ar - bol
spoken stress falls in
the penultimate syllable, and word DOESN'T end with a vowel, n, or s. has written accent
camión = truck palabra aguda
word with 3 syllables = ca - mi - on
stress falls in the
last syllable, and word DOES end with vowel, n, or s. has written accent