Introduction:
I would like this page to be a complete guide with all the necessary rules which will allow you to write the accents correctly in Spanish. Unfortunately I don't really believe that it is very easy to learn the rules and then be able to apply them. I myself find that writing Spanish accents is difficult and so I use a mixture of knowing the rules and memorising the individual words. Before I send a written text I cheat by using the spell checker although there are some cases where the accents change the meaning so beware.

Why do accents exist in Spanish?
They show the word stress when the word stress breaks the normal rules. If you understand the written accents you will be able to read Spanish out loud with correct word stress. Some words can have a change in meaning according to the accents eg. esta = this, está = it is. This concept also exists in English, compare the pronunciation of refuse = rubbish, and, refuse = choose not to do something.

What are the rules for regular word stress, i.e. words with no accents?
We use accents to show irregular word or syllable stress. Therefore first of all we should learn the rules for regular word stress (for words of more than one syllable).

Rule 1:
Words ending in a vowel; n or s are stressed on the last syllable but one.
eg: casa, libro, mesa, coche, madre, lunes, joven

Rule 2:
Words ending in a consonant (not including n and s) are stressed on the last syllable.

eg: beber papel, verdad

Now we will look at words that break the rules and therefore have an accent.
When a word does not follow the rules above we put an accent over the stressed vowel.
eg:
Words that break rule 1 (they ought to have the stress on the penultimate syllable)
comí, miércoles, cuídate, vivió, estación

Words that break rule 2 (they ought to have the stress on the last syllable)
lápiz, clímax, difícil,
(note: not many of accented words have broken rule2)

Now we look at when we have two vowels together and it starts to get complicated.
When there are two vowels together we have to consider if they are one or two syllables and if they form one syllable, which one of them should have the syllable stress?

There are two types of vowels:
Strong vowels: a, e and o
Weak vowels: u and I

Rule A When two strong vowels come together they make two separate syllables:
eg: tarea, caer, poema, peor

Rule B When two weak vowels come together they make a dipthong (one syllable):
(the stress goes on the second vowel)
eg: ruido, viuda.

Rule C When there is one strong vowel and one weak vowel together they make a diphthong (one syllable) (the syllable stress goes on the strong vowel)
eg: idiota, causa, Juan, oigo
Note: A strong and a weak vowel together make one syllable so the stress on "lengua" is on the penultimate syllable in accordance with rule 1

Words that break rule B (the stress ought to be on the second vowel)
Flúido
Words that break rule C (the stress ought to be on the strong vowel)
río, vía, oído, grúa

Are you confused yet?
You may be thinking to yourself "this is all fine and dandy but I doubt if I can remember all that" Well don't worry because neither can I. Accents will help you read correctly but for a non native they are difficult to use when you are writing.
Here are some notes which will help you:

Tip1: The most common use of the ó accent are the words that end in -ación. eg. nación, concentration There are not many words that end in -on that don't have an accent, the big exception is the third person plural preterites eg. jugaron = they played, amaron = they loved etc

Tip2: Learn the accents in the regular verbs because they will be the verbs that you use most.

Tip3: Learn the accents in of the most common words. Examples which come to mind are, aquí, más, allí, página, sí (yes not if).

Accents to show meaning

Some words which are spelt the same have an accent to show their meaning or different grammatical function.