11. course, subject

course 课程(可包括多门科目),例:a summer course
subject 科目(具体的学科)

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. 例: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. 例: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,例:Practice makes perfect.

15. class, lesson

作“课”解时,两者可以替换。
指课文用lesson,例:Lesson 6,第6课;
指班级或全体学生用class,例:Class 5,5班。