Nouns ending with the letters -ch, -sh, -s, -x, or -z

 (l. poj.) box, brush, sandwich

 (l. mn.) boxes, brushes, sandwiches

  Nouns ending with a consonant and -y

 (l. poj.) diary, baby, party

 (l. mn.) diaries, babies, parties

 Nouns ending with the letters -f or -fe

 (l. poj.) shelf, knife, leaf

 (l. mn.) shelves, knives, leaves

 Nouns ending with the letter -o

 (l. poj.) tomato, hero, kilo, potato, video

 (l. mn.) tomatoes, heros, kilos, potatoes, videos

 Irregular plural nouns

 (l. poj.) child, tooth, foot

 (l. mn.) children, teeth, feet