I read this sentence:
The sentence is supposed to mean "I don't understand why he is doing this," and I think it is grammatically correct since I found it in a japanese site ( but if it is not, please help with correcting it ? ). My question is, why is のか used in this sentence? When is のか used? I know how to use のが and のは、but のか looks new to me. Can it be replaced by のが？ ( 私は彼がこれをしているのが、分からない ).