"Ten days HAVE passed", but "Ten days IS a long time". Why?
A question for fellow English native-speakers...
"Ten days HAVE passed" (plural)
"The ten days WERE fun" (plural)
"Ten days IS a long time to wait" (singular)
"Ten days IS the limit" (singular)
A non-native speaker asked me why verb's singular in the second two, given the subject is plural. I was stumped, and had to say "because the object feels singular". Not a very useful explanation!
However *"the ten days WAS fun" or *"Ten days ARE a long time" definitely sound wrong to my ears.
How would you explain this? Is it related to the the transitivity of "be"?