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I read this sentence:

"私はなぜ彼がこれをしているのか、分からない"

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 ).

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  • Not exactly duplicate because the given link above does not explain the compound particle のか. Jun 7 '16 at 15:39
  • Snail plane's answer there covers のか. This question would help, too.
    – naruto
    Jun 7 '16 at 16:14

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