1. Spanish accents and stress syllables
Spanish Rocks
Spanish accents and stress syllables

2. What is a stress syllable
Every word has a stressed syllable, it is the one that carries the most intonation in your voice. 
For example
In the word "baby,", the stress is on the first syllable (ba) making the second syllable (by) weaker. 
Baby is pronounced as BA-by, not ba-BY. 
In Spanish there are a few simple rules as to where the stress lies in a word.  When there is an exception to these rules, a written accent is used. 
This written accent is found above vowels and is a slanted line going from left to right.

3. Accent marks look like this:
á    é    í    ó    ú
Pay attention to accent marks! Two words might be spelled exactly the same, but the presence of an accent mark can completely change their meaning.
Example:
esta means “this” while está means “is.”
Llamo means “I call” while llamó means “He, she, or you called.”
Accent marks are also used to distinguish how a word is used
dónde and donde both mean “where,” the first is used to ask a question, the second is used in statements.
Whenever a vowel carries a written accent, the stress in that word is on that syllable. 

4. Spanish is a unique language in that by just looking at a written word, without having heard it, you are able to pronounce it. 
Words are classified in three categories depending on the location of the accentuated syllable.
These are:
Aguda– the accented syllable is the last syllable
Llana– the second to last
Esdrújula– the syllable before the second to last

5. Rule # 1 (aguda)
If the word has the primary stress on the last syllable (called an oxytone), and it ends with "n", "s", or a vowel, the accent is written.
No other oxytones have a written accent, excluding exceptional cases.
Examples with accent :
pasión (passion),
limón (lime or lemon),
inglés (English),
Alá (Allah, the Arabian deity),
Canadá.
Examples without accent:
internet (no accent, stress on last syllable),
estar (one of the "to be" verbs in Spanish),
calor (heat, warmth),
Alcohol (alcohol. Remember, this has the stress on the last syllable!)

6. Rule # 2 (llano)
If the word has the primary stress on the penultimate syllable (second to last syllable called a paroxytone), and it does NOT end with "n", "s", or a vowel, the accent is written.
No other paroxytones have a written accent, excluding exceptional cases.
Examples with accent:
cárcel (jail),
árbol (tree),
González (a family name, but if written Gonzales, there is no accent.)
Examples without an accent:
ingles (groins, usually very different from "inglés"),
libro (book),
gato (cat),
perro (dog).

7. Rule # 3 (Esdrújula)
If the word has the primary stress on any syllable before the penultimate syllable (called a proparoxytone), then the accent is written.
There is no exception to this rule.
Examples:
América, México,
Pacífico (peaceful, capitalized here in reference to the Pacific Ocean).

8. Other purposes
Besides indicating the syllable in which to stress a word, written accents also serve to distinguish between two words that are spelled the same. 
Example:
"si" and "sí". 
"Si" without an accent means "if" and "sí" with an accent means "yes." 

9. A few more details
Written accents are used as a tool to make the pronunciation of written Spanish clear to the reader. 
They do not change the pronunciation of the vowel, they just put the stress on the syllable the vowel is in. 
Whenever a word has an intonation, that is an exception to the first or second rule above, the written accent portrays where to put the stress in your voice.