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Accent marks are in Spanish for two different purposes: to indicate where the stress or emphasis falls on a word when it is pronounced, and to help differentiate between identically spelled words. They are only used over vowels (á, é, í, ó, ú), and are written as a short diagonal line, from upper right to lower left.

1. The following rules regarding written accents assume you know on which syllable of a word the stress occurs. An accent mark is normally NOT required if: 
   
   1. The word ends in a vowel (a, e, i, o, u) or n or s and the stress falls on the next-to-the-last (or penultimate) syllable. Examples:
      
      

      camino		[ca-MI-no, road, ends in a vowel, and the stress falls on the next-to-the last syllable]
      hablan		[HA-blan, they speak, ends in an n and the stress falls on the next-to-the-last syllable]
      muchachas		[mu-CHA-chas, girls, ends in an s and the stress falls on the next-to-the-last syllable]

   2. The word ends in a consonant other than n or s and the stress falls on the last syllable. Examples:

      querer		[que-RER, to want, ends in an r and the stress falls on the last syllable]
      ciudad		[ciu-DAD, city, ends in a d and the stress falls on the last syllable]

2. A written accent mark IS required if the situation is not as given in the above two rules, that is, when:
   1. The word ends in a vowel (a, e, i, o, u) or n or s and the stress falls on the last syllable. Examples:
      
      

      caminó		[ca-mi-NÓ, he/she walked ends in a vowel, and the stress falls on the last syllable]
      canción		[can-CIÓN, song, ends in an n and the stress falls on the last syllable]
      verás		[ve-RÁS, you will see, ends in an s and the stress falls on the last syllable]

   2. The word ends in a consonant other than n or s and the stress falls on the next-to-last syllable.  Examles:

      mártir		[MÁR-tir, martyr, ends in an r and the stress falls on the next-to-the-last syllable]
      cárcel		[CÁR-cel, jail, ends in an l and the stress falls on the next-to-the-last syllable]

   3. The stress falls on any vowel more than two syllables from the end of the word.  Examples:
      
      

      propósito		[pro-PÓ-si-to, purpose, is stressed on the third-from-the-last syllable]
      dándomelo		[DÁN-do-me-lo, giving it to me, is stressed on the fourth-from-the last syllable]

   4. The stress falls on a weak vowel (i or u) which comes immediately before or after a strong vowel (a, e, o).  Examples:

      día		[DÍ-a, day; the stress falls on an i which is immediately followed by an a]
      actúo		[ac-TÚ-o, I act; the stress falls on a u which is immediately followed by an o]

      Note: Unless accented, the letters i and u become semi-consonants when the come in contact with other vowels. Thus, without written accents, the above words would be pronounced DYA (one syllable) and AC-tuo (two syllables).

   To sum up the above rules: Does the word end in a, e, i, o, u, n, or s? If the answer is “yes” and it is not stressed on the next-to-the-last syllable, a written accent is required. If the answer is “no” and it is not stressed on the last syllable, a written accent is required.

3. Accent marks are required on some words to differentiate them from otherwise identically spelled ones. There are three groups of these word pairs; in all cases the more emphatic word of the pair carries the written accent mark
   1. Exclamation and question words are written with a written accent mark; the corresponding relative pronouns or connecting words are not:

      ¿adónde?	(to) where?		adonde	(to) where
      ¿cómo?	how?		como	as, like, because
      ¿cuál?	which?		cual	which, as
      ¿cuándo?	when?		cuando	when
      ¿cuánto(s)?	how much, how many?		cuanto(s)	as much, as many
      ¿dónde?	where?		donde	where
      ¿qué?	what, how?		que	which, that
      ¿quién(es)?	who, whom?		quien	who, whom

      Examples:

      ¿Adónde vas?		Where are you going?
      ¿Cuánto cuesta?		How much does it cost?
      ¡Qué interesante!		How interesting!
      No sé cómo se llama.		I don't know what his/her name is. (indirect question)
      Cuando vengan, iremos al parque.		When they come, we'll go to the park.
      Digo que no es verdad.		I say that it isn't true.
      La mujer de quien hablas es la jefa.		The woman you're talking about is the boss.

   2. Several pairs of monosyllabic (one-syllable) words are similarly distinguished by the presence or absence of accent marks. The word that is more emphatic receives the written accent.

      dé		give (subjunctive of dar)		de		of, from
      él		he, him		el		the
      más		more, most		mas		but
      mí		me		mi		my
      sé		I know (saber), be (ser)		se		himself, herself, etc.
      sí		yes, indeed		si		if, whether
      té		tea		te		you, yourself
      tú		you		tu		your

   3. The demonstrative pronouns (éste, ése, and aquél, this one, that one) generally carry a written accent mark, while the demonstrative adjectives (este, ese, and aquel, this, that) never do. Note also that the neuter pronouns esto, eso, and aquello (this thing, this matter, this business) never take an accent mark since there is no corresponding adjective form.

      éste		ésta		éstos		éstas		this one, these
      ése		ésa		ésos		ésas		that one, those
      aquél		aquélla		aquéllos		aquéllas		that one, those

      este		esta		estos		estas		this, these
      ese		esa		esos		esas		that, those
      aquel		aquella		aquellos		aquellas		that, those

      Examples:

      No me gusta esta blusa; prefiero ésa.		I don't like this blouse; I prefer that one.
      Eso es estraño.		That's strange.

4. Special cases.
   1. When an adverb is formed by adding -mente to the feminine form of an adjective, the accent mark is retained, if the adjective form has one.   Examples:

      fácil		easy		fácilmente		easily
      rápido		quick		rápidamente		quickly

   2. Sólo vs. solo:

      Sólo (with an accent)	=	solamente, only (an adverb)
      solo (without an accent mark)	=	alone (an adjective).

   3. Por qué vs. porque:

      Por qué (two words, with an accent mark)	=	why (question word)
      porque (one word, no accent mark)	=	because.