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. eg: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 eg:Practice makes perfect.

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. eg: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 eg:a series of lecture on…