But then we could almost only use singular.
I think (the amount of) 3 men is enough.
I know (the number of) 10 computers is too few.
I believe (the period of) three days is too many; two days is enough already.
Sounds very strange to me.
I think 3 men are enough.
I know 10 computers are too few.
I believe three days are too many; two days are enough.
Sounds correct to me.
I'm sorry, corum, but this is my point of view.
If you want to say "the amount of", then do it.
To me it makes no sense to "imagine" words, and in the original sentence there is no mention of "the amount of".
Two days - are - enough.
Plural - are - adjective.
Another example:
Women are my weak point.
Two women are my weak point.
My weak point is women.
Would you really say, "Women is my weak point"?
I guess you won't.
So why should, "Two women is my weak point"? be correct?
"My weak point is women" -> no doubt!
Singular - is - subj. complement.
Cheers!
***** NOT A TEACHER *****
Hello, Nightmare.
*** Two days is/are enough [time] for this project.
(1) If you are taking a test, most teachers would probably expect "is."
(2) In regular, daily conversation, it does NOT matter. Many native speakers would feel more comfortable with "are." For example, an excellent teacher in this thread prefers "are," too.
*** In English Review Grammar, Mr. Walter Kay Smart says that although "is" is preferred in the following sentences, "are" is also "permissible":
Two pounds of sugar is (are) allowed to each customer.
A thousand dollars has (have) been spent.
Ten miles of concrete was (were) laid.
Forty yards of material is (are) needed.
***If you feel more comfortable in using "are," go ahead and do it. Native speakers will almost never notice any big difference.
*** BUT ***
There are a FEW times (very few) when you really should use "is":
Fifty dollars IS the price of the book. (Saying "are" would immediately mark you as a non-native speaker.)
Fifty miles IS a long way to walk. (Actually, some native speakers might actually use "are" in certain circumstances.)
Fifty dollars IS too MUCH for that. (One book says that it would be "odd" to say, "Fifty dollars ARE too MANY for that.")
***
In other words, Nightmare, you are right to say the word with which you feel comfortable. Only in a VERY few cases does it matter. Even native speakers do not agree on many sentences. Is it "2 + 2 IS 4" or "2 + 2 ARE 4"? Many books say it's your choice (although most favor "is").
***
Mr. Smith: Darling, we already have 8 children. Let's have another child. OK?
Mrs. Smith: No way! 8 IS enough!!!
(If the wife had said, "Eight are enough," that would have been correct, too. But "8 is" seems to better emphasize the number.)
***
One very good book gives a sentence similar to your "Women is/are my weak point" example:
(a) Good manners are a rarity nowadays.
(b) Good manners is a rarity nowadays.
(i) The book explains that maybe this speaker was thinking of "a rarity." That is, "A rarity nowadays is good manners."
***** Thank you *****