11. course, subject
course课程(可包括多门科目)，subject科目(具体的学科)a summer course
12. custom, habit custom传统风俗，习俗，也可指生活习惯，后接to do， habit生活习惯，习惯成自然，后接of doing. I've got the habit of drinking a lot.
13. cause, reason
cause 指造成某一事实或现象的直接原因，后接of sth./doing sth，reason用来解释某种现象或结果的理由，后接for sth./doing sth. the reason for being late
14. exercise, exercises, practice
exercise运动，锻炼(不可数)，exercises练习(可数)，practice(反复做的)练习 Practice makesperfect.
15. class, lesson
作"课"解时,两者可以替换.指课文用lesson. 指班级或全体学生用class. lesson 6; class 5
16. speech, talk, lecture
speech指在公共场所所做的经过准备的较正式的演说，talk日常生活中的一般的谈话，讲话，lecture学术性的演讲，讲课 a series of lecture on...
17. officer, official
officer部队的军官，official政府官员 an army officer
18. work, job
二者均指工作。work不可数，job可数 a good job
19. couple, pair
couple主要指人或动物，pair多指由两部分组成的东西 apair of trousers
20. country, nation, state, land
country侧重指版图，疆域，nation指人民，国民，民族，state侧重指政府，政体，land国土，国家 Thewhole nation was sad at the news.
