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My friends from Australia and Boston both say that they don't use
"effort" in the plural form. They both think that "effort" shouldn't be used as a countable noun. But I've seen many examples of "efforts."
What do you think? For example, I'd appreciate it if you could answer the following questions.

1) I did that without an effort / effort / efforts.
2) Whether he succeeds or not depends on his effort / his efforts.
3) They made a lot of effort / efforts to get the job done.

Are there any personal or official rules to follow?
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I don't agree that effort can only be used as an uncountable noun, though in many instances it is.
Looking at your sentences:
1) I would say I did that without effort or I did that without any effort or I did that with no effort. I wouldn't use the plural there.
2) Either is possible, but the meaning changes slightly.
Whether he succeeds or not depends on his effort (how hard he works).
Whether he succeeds or not depends on his efforts (what he does to try to achieve success (which could include how hard he works)).
3) Either is possible, but the meaning changes slightly.
They made a lot of effort to get the job done (they worked hard).
They made a lot of efforts to get the job done (they tried several times).
One sense of the word is always countable:
He enters the pumpkins he grows in competitions, and has won several prizes for his efforts (i.e. the pumpkins he has grown).
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teacherJapan They both think that "effort" shouldn't be used as a countable noun.
If this means "shouldn't ever be used", it is a strange opinion. The word "efforts" is in common use, in the right context.
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Comments  
Thank you very much for your great examples. Your explanation is always clear and to the point!
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