When forming the plural of Greek nouns, the following rules apply:

Masculine nouns with an -ος ending form the nominative plural with an -οι ending.
Masculine nouns ending in -ας and -ης build the nominative plural with an-ες ending.

💡 Examples:
Singular
ο φίλος – the friend
ο χειμώνας– the winter
ο καθηγητής– the teacher/ professor

Plural
οι φίλοι – the friends
οι χειμώνες – the winters
οι καθηγητές – the teachers/professors

Feminine nouns with an -α and -η ending form the nominative plural with an -ες ending.

💡 Examples:
Singular
η φίλη – the (female) friend
η καθηγήτρια – the (female) teacher

Plural
οι φίλες – the friends
οι καθηγήτριες – the (female) teachers

Neuter nouns with an -ο ending form the nominative plural with an -α ending.
Neuter nouns with an -ι ending form the nominative plural -ια ending.
Neuter nouns with an -μα ending form the nominative plural with an -ματα ending.

💡 Examples:
Singular
το κινητό – the mobile phone
το σπίτι – the home
το μάθημα – the lesson

Plural
τα κινητά – the mobile phones
τα σπίτια – the homes
τα μαθήματα – the lessons