What are the accent placement rules in Spanish?

Question

This is a canonical question / Esta es una pregunta canónica

Some people have trouble determining when accent marks (tildes) should be used and where.

What are the rules for accent placement in Spanish? How do you determine whether the vowel on the stressed syllable should have an accent mark or not?

Answer 1

There are three main categories of words in Spanish, according to what syllable the stress falls on: agudas, llanas o graves, and esdrújulas. Let's see what these terms mean.

Words are divided in syllables. In order to know which syllable gets a tilde, or written accent mark, you need to count the syllables. The counting goes from the last syllable towards the beginning of the word:

0. One-syllable words: One-syllable words never have a tilde, except in the ambiguity-breaking cases discussed at the end of this answer.

1. Agudas: These are the words that have the stress on the last syllable. They get the tilde if they end in -s, -n, or vowel.

café, posición, estás, etc.

2. Llanas o graves: The words falling into this category have the stress on the penultimate (second to last) syllable. They get the tilde when they don't end with -s and -n or vowel (the opposite situation from the first category I listed).

cónsul, López, carácter, etc.

3. Esdrújulas: These words have the stress falling on the third to last and fourth to last syllable. These words are always written with a tilde.

práctica, próximo, jóvenes, diciéndoselo, etc.

Other rules:

• You put a tilde also on words that have hiatus formed by strong vowels (a, e, o) and weak vowels (i, u), such as:

período, Raúl, actúan, filosofía, etc.

Note that the case of the fourth-to-last syllable can also be called by a specific term, "sobreesdrújula." Example: Less common are the words with the stress falling on the fourth-to-last syllable, such as diciéndoselo.

• Adverbs ending with -mente get the accent when they come from an adjective that has it: rápido → rápidamente, cordial → cordialmente. (Source)

• Monosyllables require an accent mark to differentiate from others which would be homonyms. The variants which have the accent mark are usually stressed when they are pronounced inside a sentence:

  • tú (subject pronoun) – tu (possessive pronoun)
  • él (pronoun) – el (article)
  • dé (present subjunctive of dar) – de (preposition)
  • mí (prepositional/stressed pronoun) – mi (possessive)
  • sí ("yes") – si ("if" or the musical note)
  • más ("more" or "plus") – mas ("but")
  • sé (present tense of "saber") – se (reflexive/reciprocal/indirect object pronoun)
  • té ("tea") – te (unstressed pronoun)
  • interrogative (or exclamative) pronouns: cuándo, qué, cómo, dónde, cuál, quién – non-interrogative corresponding conjunctions: cuando, que, como, donde, cual, quien

• Some special words can be accented to distinguish the meaning:

  • demostrative pronouns éste, ése, aquél, éstos, ésos, aquéllos, ésta, ésa, etc. – demostrative determinants este, ese, aquel, estos, esos, aquellos, estas, esas, etc.
  • sólo (only/just) – solo (alone)

Answer 2

I support the answer by Alennano. However, I will write an answer that takes a slightly different point of view, intended to be especially practical and useful for a Spanish learner or a heritage speaker who is unsure about written accent marks:

If you are reading written text and would like to know where to put the stress:

Rule 1. If there is a tilde:

• the stress will go where you see the tilde. (lápiz, académico)

Rule 2. If the word has no tilde mark, and it ends in n, s or vowel:

• The stress will go on the second-to-last syllable. (hablan, hablas, habla, persona)

Rule 3. If the word has no tilde mark, and it ends in a consonant other than n or s:

• The stress will go on the last syllable. (personal, verdad, hablar, capaz, doblez, islam)

The flip side of the above comes next.

If you would like to write down a word based on how it sounds:

First, write the letters of the word. Then, applying Rules 2 and 3 from above, pronounce what you've written. If the pronunciation comes out right then you don't need to write an accent mark. Otherwise, you do.

For the fine points having to do with diphthongs and semantic differentiation, please see the answer by Alennano.