1.变内部元音
foot-feet
man-men
mouse-mice
tooth-teeth
woman-women
2.词尾加-en
child-children
ox-oxen
3.形式不变,即单复数同型(通形名词)
deer-deer
fish-fish
means-means
series-series
sheep-sheep
4.加-es(是加-s的特殊情况)
match-matches
tomato-tomatoes
5.去y改i加-es或去f改v加-es(亦为加-s的特殊情况)
leaf-leaves
6.外来词的不规则变形
ovum-ova
locus-loci
basis-bases
(不少外来词有两种复数形式)
focus-foci/focuses
cactus-cacti/cactuses
fungus-fungi/funguses
我知道的就那么多了...