I have a question connected with the articles. I found this sentence in a book:
'London and Warsaw are capital cities.'
I want to know if it would be correct to use 'the' before the word capital in this sentence.
No, it wouldn't.
This sentence says that L. and W. are examples of capital cities, examples of objects called 'capital cities' - not any special capital cities.
It's like saying
John and Betty are students. Why say 'the students' if you just want to make it clear that they are not drivers or schollchildren or university teachers?