The formation of the plural in Italian is not as straightforward and easy as it is in Spanish or English, where just an s must be added to form the plural (dog => dogs, perro => perros) and the few exception (woman => women) can be counted with one hand. In Italian the basic rules are a little bit more complicated, and there are more exceptions to the basic rules.
Let's start with the basic rule.
Words, which end in -a, form the plural on -e (and are feminine in general)
la casa => le case
the house => the houses
la donna => le donne
the woman => the women
la finestra => le finestre
the window => the windows
la ragazza => le ragazze
the girl => the girls
Words, which end in -o, form the plural with -i (and are masculine in general)
il giorno => i giorni
the day => the days
il libro => i libri
the book => the books
l' anno => gli anni
the year => the years
il ragazzo => i ragazzi
the boy => the boys
Words, which end in -e, form the plural in -i (in these case the gender must be learnt as well)
