Neil Whitfield gives an answer on his ESL blog here. It depends on whether the quantity is countable or not.
[C]ountable nouns form plurals; mass/uncountable nouns don’t. This gets a little more complicated because some nouns may be either countable or mass/uncountable, depending on how they are being used. “Wheat” for example may be both: ten kilos of wheat is uncountable; several types of wheat is also uncountable; there are several wheats used in this mix is countable.
OK, with countable nouns: I would say 0.1 apples for grammatical reasons, though I agree it is not logical! I guess you could say 0.1 of an apple just as you say one-tenth of an apple If the number one is used, whether it is +/-1, the following noun will be singular. So it would be -1 apple. We’re talking grammar, not logic; and yes we say zero apples, probably because zero is thought of as a number that is not one, even though zero is neither singular nor plural logically.
So it looks like grammatically (perhaps not mathematically) the following
- 0.3 = point-three apples (uncountable)
- 1/3 = one third of an apple (countable one in fraction)
- 2/3 = two thirds of an apple (countable more than one third)
- 0 = zero apples, no apples (uncountable)
- 1 = one apple (countable)
- 3 = three apples (countable)
However, (Caveat, I'm not a grammar professional)
- -1 = negative one apples (uncountable, you can't have negative of something real, like money. This is a mathematical concept only, hence plural would be ok.) This feels like
- -1 = minus one apple (countable. "After they broke into his house, he was left with minus one apple." The number of apples stolen is countable.) This feels like
I hope that clarifies it a bit more. They're both appropriate in different circumstances. Hooray for English!
I'll add just a bit more. If the word
one can be replaced by an article (
a/an/the) then you should use the singular.
The temperature is minus one degree Celsius.
The temperature is minus a degree Celsius.
However, the following is kindof awkward.
Ten subtracted from nine is negative one degrees Celsius.
Ten subtracted from nine is negative a degree Celsius.
So you should probably use the plural.