How do accents (tildes) work in Spanish?

Many Spanish words have accent marks (e.g. móvil, habló or fantástico). While this seems odd for english speakers, the rules followd in Spanish are suprisingly straightforward!

Accents mark word stress.  This can seem ambiguous in English (compare re-search and re-search), but Spanish words always indicate how they should be stressed. They usually falls into one of three grouops:

(1) Words stressed on the last syllable carry an accent provided they end in a vowel or the consonants -n or -s (e.g. comió, pagarás); no accent is present with other consonants (e.g. comenzar).

(2) Words stressed on the penultimate syllable carry an accent whenever they in a consonat other than -n or -s (e.g. mármol), but not otherwise (e.g. casa).

(3) Words stressed on the second-to-last syllable always carry an accent mark (e.g. sílaba).

These basic rules always work, provided the stress is known (e.g. you know how to say the word or have heard it). On the other hand, once you know these rules, you can always correctly stress a word when you read it!