there isn't an special rule with this letter... all vowels have the same rules for having an accent on them. if the last syllable has the stress of the word must be accented in this syllable if the word finishes on vowel, n or s. This kind of words are called "agudas". if the previous syllable has the stress must be always accented except if the word finishes on vowel, n or s. This words are called "llanas" if the stress is in another syllable must be always accented without exception. This words are called "esdrújulas" or "sobresdrújulas". There is another rule for accenting vowels when you want to break a diphthong (vowels combined with i or u). In this case you must to put an accent on "i" or "u". If this diphthong has the stress of the word the other vowel of the diphthong will be accented depending of the other rules... And there are another rules in questions and indirect questions... where I think you have the doubt as the case as cómo? dónde? cuándo? etc... always will be marked when they aren't an adverb... have a look at this link http://forum.wordreference.com/showthread.php?t=334967 where giving a good explanation about. I hope to have explained clearly... you can have a look at this link too http://spanish.about.com/od/spanishpronunciation/a/stress_accent.htm

[Copy of earlier answer from duplicate account now deactivated] All vowels are accented according to the same rules: Rule 1: Unless the vowel occurs in a STRESSED syllable, it CANNOT be accented. To occur in a stressed syllable, therefore, is a NECESSARY condition, but NOT a SUFFICIENT one; further rules apply: Rule 2: If the stressed syllable is LAST in its word and its vowel is syllable-final, it MUST be accented, cf. "ganó", "gigoló", "desempaquetó", etc. Rule 3: If the vowel of a word-final stressed syllable is followed by /n/ or /s/, it MUST be accented, cf. "jamón", “ilusión”, "adiós", etc. If it is followed by any other consonant, it CANNOT be accented, cf. "motor" (not *"motór"), "mongol" (not "*mongól"), etc. Rule 4: If stress falls on the penultimate syllable, its vowel is accented UNLESS the word ends in a bare "s" e.g. “árbol”, “trébol”, "píxel", "cárter", "cóndor", “búfer”, “tándem”, "ántrax", "bíceps", but very few (mostly foreign) words satisfy such conditions. Hence, "goma", not "*góma", "Roma", not "*Róma", and "loco(s)", "tono(s)", "broma(s)", "sopa(s)", etc. This rule does NOT apply when it is necessary to distinguish words with different functions, e.g. "como" (= like) from "¿cómo?" (= "how?"), both /KO.mo/, or "solo" (= alone) from "sólo" (= only), both /SO.lo/. Rule 5: If stress falls on the antepenultimate syllable, its vowel MUST be accented, as in many words from Greek, e.g. "psicólogo", "antropólogo", "halógeno", "lógico", "tópico", or Latin, e.g. "óvulo", "lóbulo", "cómodo", etc. Rule 6: Since syllables preceding the antepenultimate never COUNT as stressed (even if they are!), their vowels are NEVER accented, either. Thus, although we may say /PRO.to.in.doeu.ro.PE.o/, we write “Protoindoeuropeo”, not “*Prótoindoeuropeo”. Rule 7: The vowels of monosyllables are not accented, even if stressed, unless, again, two words must be distinguished (e.g., "si" from "sí", "que" from "qué", etc.). Best, :-)

Regla Las palabra agudas sólo se acentúan, cuando terminadas en vocal , o en las consonantes n, s por ejemplo mamá, café, corazón... Buena sorte!

About Ruben said, I give you some examples: - Agudas: to-mó, co-rrí, dur-mió, pan-ta-lón, ca-mión, vai-vén, can-ción, cor-cel - Graves o llanas: lá-piz, rí-o, ca-rác-ter, ma-ri-po-sa, ins-tru-men-to, ca-pri-cor-nio - Esdrújulas: Es-pec-tá-cu-lo, con-tem-po-rá-ne-o, pe-rió-di-co - Sobreesdrújulas: En-tré-ga-me-lo Finally when there is a letter “o” between two letters you has to accent, whether “o” is between two numbers, you don’t accent. Example: - Blanco o negro - 4 ó 6