As in English there are certain rules to be followed when forming the plural of a noun.  The vast majority of Spanish nouns form their plurals by adding "s", by adding "es" or by no change at all from the singular. 

Words requiring the addition of "s"

a) Nouns ending in an unstressed vowel:  

singular plural
la cama bed las camas beds
el huevo egg los huevos eggs
la tribu tribe las tribu tribes

b) Nouns ending in stressed "e" including words of one syllable ending in "e":

singular plural
el pie foot los pies feet
el clisé cliché los clisés clichés

c) Nouns ending in an unaccented vowel:

singular plural
la caja box las cajas boxes
el palo stick los palos sticks
la casa house las casas houses
Notable exceptions to the rule:
papá father papás fathers
mamá mother mamás mothers
sofá sofa sofás sofas
menú menu menús menus
tisú tissue tisús tissues

  Words requiring the addition of "es":

a) Native nouns ending in a consonent other than "s":

singular plural
el avión airplane los aviones airplanes
el color color los colores colors
el rey king los reyes kings

b) Nouns ending in a stressed vowel plus "s":

singular plural
el autobús bus los autobuses buses
el mes month los meses months

Note: Although "mes" does not have an accented (or stressed vowel) because it is only one syllable it is understood to be stressed and does not require an accent mark.  Therefore similar one syllable words would follow this rule.


Orthographic changes: Some words follow the general rule in terms of pronunciation, but a change in either a letter or an accent mark is needed. In plurals of words that end in -z, the -z changes to -ces for the plural. And if the addition of -es would change which syllable gets the accent, an accent is either dropped or added.


Examples: el juez (the judge), los jueces (the judges); una vez (once), dos veces (twice); el inglés (the Englishman), los ingleses (the Englishmen); la canción (the song), las canciones (the songs); el examen (the exam), los exámenes, the exams.