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…
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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常和困难连系，前面的动词常为think about, solve, raise，question常和疑问连系，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.