非常好的语法复习材料。可以给教师备课用，也可以给学生自学用。
1.规则变化:加-s, -es; 辅音字母加y结尾的去y加ies;把f/ fe变成ves;
(1)keys , three Henrys; stomachs; safes; beliefs; proofs; roofs; gulfs; chiefs; heroes; potatoes; tomatoes; Negroes; dingoes
(2) 缩写,数字,字母等在后面加s或’s都可,如: some VIPs (VIP’s); in his 50s / 50’s; in the 1990s / 1990’s; There are two t’s in the word “letter”.
