• 一般情况:加-s(book→books, pen→pens)
• 以s/x/ch/sh结尾:加-es(bus→buses, box→boxes)
• 以“辅音+y”结尾:变y为i加-es(city→cities, baby→babies)
• 以“o”结尾:有生命加-es(potato→potatoes),无生命加-s(photo→photos)
• 以“f/fe”结尾:变f/fe为v加-es(knife→knives, leaf→leaves)
• 单复数同形(sheep→sheep, deer→deer)
• 特殊变形(man→men, woman→women, child→children)
• 外来词保留原复数(cactus→cacti, phenomenon→phenomena)
4. 用法:可与a/an、数词、many、few等连用