Ends with -fe	Change f to v then Add -s	knife knives life lives wife wives
Ends with -f	Change f to v then Add -es	half halves wolf wolves loaf loaves
Ends with -o	Add -es	potato potatoes tomato tomatoes volcano volcanoes
ends with -us	Change -us to -i	cactus cacti nucleus nuclei focus foci
ends with -is	Change -is to -es	analysis analyses crisis crises thesis theses
ends with -on	Change -on to -a	phenomenon phenomena criterion criteria
ALL KINDS	Change the vowel or Change the word or Add a different ending	man men foot feet child children person people tooth teeth mouse mice
Unchanging	Singular and plural are the same	sheep deer fish (sometimes)

When you are sure that you understand the lesson, you can continue with the exercises.