I was surprised to find out that you should write:
I waited for a torturous three days.
Instead of:
I waited for torturous three days.
Why is this? "Days" is plural ...
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Sign up to join this communityI was surprised to find out that you should write:
I waited for a torturous three days.
Instead of:
I waited for torturous three days.
Why is this? "Days" is plural ...
In phrases that follow this pattern:
a {modifier} {number} {plural noun relating to time}
{plural noun} is understood to be a duration.
She spent a nervous thirty minutes waiting to be interviewed for the job.
We spent a pleasant two weeks at the lake.
He worked a grueling three years in the desert.
In general, dates & times, lengths and weights1, amount of currency and Maths expressions are not considered plural even if they use plural forms of words. Like these
Three days isn't a long time.
Three days aren't a long time.Ten miles is too long to walk.
Five kilograms isn't heavy for him to lift.
Ten miles are too long to walk.
Five kilograms aren't heavy for him to lift.Twenty pounds is too little to buy this.
Twenty pounds are too few to buy this.Two plus three is five.
Two plus three are five.
So, in your case, "tortuous three days" is considered as "a duration of time", so even if the word "day" is in its plural form, we do not consider the phrase as a plural form.
If you really want to treat it as plural, you can say
I waited for three tortuous days.
I waited for tortuously three days.
In the upper way "days" will be treated as a regular noun, losing its meaning of "duration of time" because an adjective is inserted in between the number and "days".
Besides, as minnmass mentioned in comments, this rule breaks down if the measure is a part of a whole. For example:
Three days isn't a long time, unless two are rainy.
The first part of the sentence follows the rule, but the latter part doesn't, as "two" is part of the whole ("three days").
References: This and this on English.SE. A Google search query is also preferred.
1. Actually all physical measurements are never considered plural. For example, "10 Amperes is a large current" and "100 degrees Celsius is the boiling point of water" and "101325 Pascals is the standard atmosphere pressure".