If the word ends with a vowel, –n or –s, the stress is on the penultimate syllable. Words with the penultimate syllable stressed are called llanas and they form the majority:
metro [‘me–tro]
canon [‘ca–non]
amigos [a–‘mi–gos]
If the word ends with a consonant, except –n, –s, the stress is on the last syllable. Such words are called agudas:
hotel [o–‘tel]
tractor [trac–‘tor]
If the word has an accent mark (la tilde), it means the stress is on the marked syllable. Accent mark is placed in the words that do not follow the reading rule, and the words where the stress in on the antepenultimate syllable. Such words are called esdrújulas:
crédito [‘cre–di–to]
analgésico [a–nal–‘xe–si–co]
The stress can radically change the meaning of the word, compare:
hablo – I speak
habló – she spoke
seria – serious
sería – it would be
That is why it is so important to place the accent mark correctly in Spanish.