In the following sentence "They are determined to fight for what they believe (is/ are) their rights. Should I use singular "is" or plural "are"? It seems to me "what" is the subject in the noun clause but the textbook uses "are".
I'd only use are in this sentence since the word 'what' refers to 'rights'.
To me it's the same logic as in these sentences:
What do they believe their rights are?
They know what their rights are.
What are their rights?