English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.
Sign up to join this community
I don't understand why the verb of the following sentence is has, not have. Doesn't some degree of modify sadness and depression? If so, is sadness and depression regarded as a single concept?
Yet, it seems that some degree of sadness and depression has been far more accepted in previous historical ages than is the case today.
Well, I think you answered your own question.
... is sadness and depression regarded as a single concept?
I can't say if that is a single concept without actually reading the book and how it is defined there. If "sadness and depression" is a single concept/idea/variable then it will take the singular verb "has". If they are two different/separate things then they will take the plural "have".
One way to check this is by seeing if the book uses the terms separately. For example, a comparison of this kind may tell you if they are different concepts:
Almost everyone experiences sadness to some degree at some point in life. It does not affect a person's ability to carry out day to day activities. In contrast, depression, which affects fewer people, is a dreadful disease - it will destroy you mentally and physically.
I just made that up.
The choice of has/have depends on whether the author "thinks of sadness and depression as two distinct things, or two components of a composite 'medical response" (see FumbleFingers' response here).
Perhaps the easiest way to explain this is by using the example of "Fish and Chips". If you talk about "fish and chips" as one dish (which it is), then it will always take the singular verb "has/is".
Take a look at these two questions in ELL for more explanation: Grammar on A and B “is” or “are” and “Macaroni and cheese (is/are) on the lunch menu”
