As you know the words are formed by sillables, and each word has and strong one, listen to these audio file to see what i am talking about:
se-MÁ-fo-ro (traffic light) click here and listen to these words
well as you can see some of those words are marked with a(´) and I am sure that you are asking yourself why some of those words are marked and some are not?? let's go to see it:
There are three kind of words, the words where the strong sillable is the last one like camión, vagón...which are called "palabras agudas", the words where the strong sillable is second to last like: difícil,patata..which are called "palabras llanas"and the words where the strong sillable is the third to last like: semáforo which are called "palabra esdrújula" (esdrújula is a palabra esdrújula it self: es-drú-ju-la)
Let' start with the easy one: all the esdrújulas words must be marked on the vowel of the strong sillable!!!
the agudas words must be marked when the word ends in N, S or a VOWEL
the llanas words must be marked when always if the word ens with a consonant and consonant+S(bíceps(bicps))
There is another kind of words, like when we make the conditional ex: caminaría(ca-mi-na-rí-a) that is llana and ens with a vowel but we also marked it to show that when we tallk we split the last sillable(we call at this exeption"diptongo"