Guide for accent marks
First, you need to know where to put stress when there is no accent. In most cases, the stress goes on the second to last syllable:
"Yo hablo" "Tú comes"
The exception, is when the word ends with any consonant other than 'n' or 's'. The most common one you will see will be words ending in 'r' because all "to verbs" (comer, correr, poder, querer) end with 'r':
"Quiero hablar", "Él quiere comer" or the word "lugar" (place), "la velocidad", "el bistec".
The thing that accents do, is change where the stress is. The word "cañón" has an accent there so you know not to put the stress on the first (second to last) syllable, but rather the last syllable, where the accent is.
Now, what if we make "cañón" plural? The plural is "cañones". Where did the accent go? Well, now we added another syllable, making the "o" the second to last syllable, which is where we put the stress.
It's the opposite with the word "joven". The stress is on the second to last (first) syllable, "jo". When we make it plural, it becomes "jóvenes". We added another syllable, so we have to clarify that the stress isn't on "ve", the second to last syllable, but rather the first syllable, "jo".
Hope I helped! Remember that I am not a native so I may have made some mistakes or forgot some things, so please tell me what I missed/screwed up!
A note: "The exception is when the word ends with an "r"
Actually the 'exception' is if a word ends in any consonant other than 'n' or 's.' Then the stress is on the final syllable. pared, musical, velocidad, bistec, calidad, nacionalidad, mural, etc.
Here's an informative link: http://faculty.weber.edu/tmathews/grammar/marks.html
I use the rule of EGA
E for esdrújula (and sobreesdrújulas) are accented on the third from the last syllable (antepenúltima) and they all have a written accent mark G for grave (sometimes called llana) are stressed on the next to the last (penúltima) syllable and ONLY have a written accent if the word --not the syllable -- ends in a consonant that is NOY "n" or "s" A for aguda and are accented on the last syllable (última) and they will have a written accent mark if they end in a vowel or "n" or "s"
I use a chart called EGA and divide it vertically and put all for E ; consonant that is not "n" or "s" for G; and a vowel or "n" or "s" for the A column. Pronounce the word and place in as an E or G or A and apply the rule. One of my students that was in elementary school in Costa Rica taught me this and she was taught it in first grade.
¡OJO! there are acentos prosódicos that are exceptions.