mankind 集合名詞

Mankind is an uncountable noun. It is never used as a countable noun.

Mankind = all humans considered as a single group. (i.e. The whole of human race, including both men and women). Some people avoid using this word because they think it is offensive to women, and they use humankind (uncountable noun) instead.

Example:
Someone once said that the invention of television was the greatest single disaster in the history of mankind.

A nuclear war would be a threat to all mankind.

Human or human being (countable noun) = a person (human beings ~ plural)

其實 Man 有另外一解釋
Man = a person, either male or female

“All men are created equal”. ~ quotation from Thomas Jefferson (including males and females)

題外:
Mankind 大多數人認識, humankind 比較沒有這麼人識

美國少數的人將傳統的英語改丟, 有時….
Postman ~ mail carrier
Policeman ~ police officer
Salesman ~ salesperson
Chairman ~ chairperson
Milkman ~ ??

Workmanship ~ craftsmanship

Ususal 人類 is "Human being "
and Mankind now US change it to other word it, by it is sexual race.
because it is combine man -kind.
therefore, we ususal use Human being for both sexual use

基本上只作不可數名詞

google search 亦找不到mankinds的例子