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Hi. When can we use the words "man" and "men"? Can they be used in place of each other?
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'Man' works like any other countable noun:

One man, two men
One bicycle, two bicycles.

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Hi. Thank you. I think the word "man" can refer to something to the "humankind." I tink we can use either word in limited contexts in place of each other.
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Nonetheless, their numbers remain different:

Men have lived on Earth for more than a million years.
Man has lived on Earth for more than a million years.
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are u sure to use "a" before million years? Should it be million years not a million years?
'A' is necessary there.
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Mister MicawberMen have lived on Earth for more than a million years.
So have women.