Spanish nouns change their number. The main rule for forming plural nouns is as follows:
If a word ends in a vowel, the plural is formed by adding –s:
la casa (house) – las casas (houses)
If a words ends in a consonant, the plural is formed by adding –es:
el reloj (watch) – los relojes (watches)
Words that end in –s, only if the last syllable is not stressed, do not change in plural:
el cumpleaños (birthday, birthday party) – los cumpleaños (birthdays, birthday parties)
You should remember also that the formation of plural nouns in some cases may lead to spelling changes. The words that end in –z, change it into –c–, to preserve the sound [θ]:
la vez (time=occasion) – las veces (times=occasion)
el lápiz (pencil) – los lápices (pencils)
If a new syllable appears, the graphic accent may appear or disappear to preserve the position of the stressed syllable:
el andén (platform) – los andenes (platforms)
el joven (young man) – los jóvenes (young people)