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I understand that certain nouns (collective nouns?) that do not have a plural word. E.g. aircraft, rain and perspiration. However, I come across such usage, specially in Asia. "Many aircraftS are taking off and landing at this airport." or "Beads of perspirationS fell from his forehead".
Are there actually such words as aircraftS and perspirationS?
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I consider both unacceptable.
HvplI understand that certain nouns (collective nouns?) that do not have a plural word. E.g. aircraft, rain and perspiration. However, I come across such usage, specially in Asia. "Many aircraftS are taking off and landing at this airport." or "Beads of perspirationS fell from his forehead".
Are there actually such words as aircraftS and perspirationS?
Hello Hvpl

Welcome to this Forum. I too am an English learner from an East Asian country.

As for perspiration, my English-Japanese dictionary says "perspiration" is sometimes used as a countable noun, but I think that usage is very rare. Google gives 5,388,000 English pages for "perspiration" but only 16,000 pages for "perspiration". But there seems some people who say "profuse perspirations" on the analogy of "profuse tears".

"Aircraft" is commonly used as a collective noun but it is sometimes used as a countable nouns.
(EX) President Roosevelt said that the United States would build 6000 military aircraft.
(EX) The United States contemplated building an aircraft the size of a football field.
(EX) We made a decision to send aircraft -- heavy cargo aircrafts to help in the rescue operations.

paco
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Aircraft is a countable noun, not a collective noun. Its plural form is also aircraft.

For example, an aircraft, two aircraft, a sheep, two sheep.
"Aircraft" can be singular and plural forms for a countable noun "aircraft", but it also can behave like a collective noun.

paco
http://en.wiktionary.org/wiki/aircraft (aircraft or aircrafts is okay)

http://en.wiktionary.org/wiki/perspiration (perspiration is only okay)
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No, Wiktionary said "aircrafts" is incorrect, but that it is becoming common to hear it pronounced that way. "Aircrafts" is not okay. Same goes for spacecrafts or watercrafts...or sheep.
These nouns without plurals can put into the plural in certain specific contexts, for example:

The rains came early this year. (This is a well-established and frequently heard usage.)

Words like aircraft and perspiration are difficult to put into plural form, but they can be forced into it in certain very specific contexts:

Because of the erupting volcano and the resulting hazardous flying conditions, aircrafts from many destinations started to land at the remote island airport. (This unusual, emergency situation enables the unconventional usage.)

Research scientists at deodorant manufacturers test all kinds of perspirations from all over the world. (Again, the unusual situation enables the unconventional usage.)