1. clothes, cloth, clothing
clothes统指各种衣服，谓语动词永远是复数，
cloth指布，为不可数名词
clothing 服装的总称，指一件衣服用
a piece of, an article of
2. amount, number
amount后接不可数名词，
number后接可数名词
a number of students
3. sound, voice, noise
sound自然界各种各样的声音，
voice人的嗓音，
noise噪音
I hate the loud noise outside.
4. 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
5. exercise, exercises, practice
exercise运动，锻炼（不可数），
exercises练习（可数），
practice（反复做的）练习
Practice makes perfect.
6. officer, official
officer部队的军官，
official政府官员
an army officer
7. work, job
二者均指工作。
work不可数，job可数 a good job
8. cook, cooker
cook厨师，cooker厨具
He is a good cook.
9. problem, question
problem常和困难联系，
前面的动词常为think about, solve, raise，
question常和疑问联系
多和ask, answer连用
10. a number of, the number of
a number of许多，谓语动词用复数。
the number of…的数目，谓语动词用单数。
The number of students is increasing.
11. chick, chicken
二者均可指小鸡，
chicken还可以当鸡肉
The chicken is delicious.
12. trip, journey, travel, voyage
travel是最常用的，
trip指短期的旅途，
journey指稍长的旅途，
voyage指海上航行
a three-day trip
13. in front of, in the front of
in front of范围外的前面
in the front of范围内的前面
In the front of the room sits a boy.
14. three of us, the three of us
three of us我们（不止三个）中的三个，
the three of us我们三个（就三个人）
The three of us---Tom, Jack and I went to the cinema.
15. by bus, on the bus
by bus表手段，方式，不用冠词，
on the bus表范围
They went there by bus.
16. for a moment, for the moment
for a moment 片刻，一会儿，
for the moment暂时，一时
Thinking for a moment, he agreed.
17. in a word, in words
in a word总之，一句话，
in words口头上
In a word, you are right.
18. in place of, in the place of
in place of代替，
in the place of在…地方
A new building is built in the place of the old one.
19. go to sea, by sea, by the sea
go to sea当海员，出航，
by sea乘船，由海路，
by the sea在海边 go by sea
20. the doctor and teacher, the doctor and the teacher
the doctor and teacher
指一个人，既是医生又是老师，
the doctor and the teacher
两个人，一个医生和一个老师
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