There are easy‐to‐understand rules explaining why accent marks are used. To start: There is only one kind of accent, only one accent (if any) per word, and it can only be placed on a vowel, never a consonant.
Because the main reason for using an accent mark is to stress a syllable that would not naturally be stressed, you must be prepared to determine which syllable would be stressed naturally. The natural stress of a word follows two simple rules:
If a word ends in any consonant other than n or s, the natural stress will be on the last syllable.
If a word ends in a vowel or the letter n or s, the natural stress is on the next‐to‐last syllable.
If the word is supposed to be pronounced with the stress somewhere other than the rules require, a written accent mark indicates which syllable to stress.
The main purpose of writing an accent mark is to indicate that this particular word is supposed to be stressed somewhere other than the syllable where it would be stressed naturally if it followed the rules. This leads to some predictable rules within the rules. For example, there are hundreds of words that are cognates of English words that end in ‐tion like “nation,” “liberation,” or “condition.” These words end in ‐ción in Spanish: nación, liberación, and condición. The rule states that a word that ends in an n has the natural stress on the next to the last syllable, but these words are supposed to be stressed on the last syllable as opposed to the stress on the first syllable in English, so an accent mark is written on the last vowel, which is the o.
If a syllable is added to the end of a word, the natural stress of the word is changed. This happens when you make a word plural by adding ‐es. The accent mark on a word like nación served the purpose of placing the stress on the o, but the plural word ends in s, so the natural stress would already be on the next‐to‐last syllable, which is now the o. Thus, you must remove the accent mark on the plural form of a word ending in ‐ción because the stress is naturally on the o in ‐ciones.
Understanding diphthongs and strong and weak vowels becomes important when you are trying to decide whether a word needs a written accent mark. Remember that one strong and one weak vowel together create one syllable. For example, the word iglesia ends in the vowel a so the stress naturally falls on the next to the last syllable, the e, according to the general rule. Thus, it does not carry a written accent. It is common, however, to see an accent on the ‐ía ending. That is because an accent mark can be used to indicate that the softer vowel is to be pronounced as well as the stronger. This creates two separate syllables and the stress will be on the accented syllable.
Understanding these rules helps you not only to know whether to write an accent when spelling a word that you hear, but also to pronounce a word that you are reading with the stress on the correct syllable.
Another reason for writing an accent mark has nothing to do with pronunciation. It is used to differentiate between two words that are otherwise spelled the same but have different meanings. For example, the word sí means “yes” and the word si means “if.” Although the accent makes the spelling different, only the context of a spoken sentence will provide a clue as to which of the two words is appropriate.