Words in Spanish have a relatively predictable pattern of stress and a reasonably simple way of knowing whether written accents are needed.
Words that end in a vowel, -n, or -s are naturally stressed on the next to the last syllable. For instance, casa, casan and casas are all stressed on the next to the last syllable, and since these words are pronounced the way one expects they require no written accent mark.
Words that end in consonants other than -n or -s have a natural stress on the last syllable. Ciudad, reloj, marfil, hablar, hacer, partir, and complot are all stressed on the final syllable and do not require a written accent.
Then, one might ask, when is the written accent mark required in Spanish? If one knows how a word is pronounced, the two preceding rules are most of what one needs to know. If, for example, a word is pronounced differently from what these two rules predict, an accent mark must be added on the appropriate vowel of the stressed syllable.
* Note that words that end in -an, such as galán and imán, etc., and others that end in -ción, such as lección, acción, and canción require the written accent, as expected. In the plural, however, with the addition of -es they acquire another syllable, and the previously mentioned words lose their written accent mark: galanes, imanes, lecciones, acciones and canciones.
There are certain words whose written accent mark has nothing to
do with pronunciation, however, and one must simply memorize them.
They are common word pairs such as de / dé [of,
from (preposition) / give (present subjunctive)] and
aquel / aquél [that (demonstrative adjective)
and that one (demonstrative pronoun)] which require a
written accent mark to distinguish one from the other.