It seems that we must use the verb "to sing" to complete each of the sentences. We can do it in the following manner:
1. Which song does Robbie Williams sing with Nicole Kidman?
2. Who sings I can't get you out of my head?
- Both sentences are interrogative, but we will use the verb "to sing" differently in each of them.
- In the first sentence, the question word is "which". "Which" is not the subject of the sentence - "Robbie Williams" is.
- For that reason, we must place an auxiliary verb before the subject. Since it is a third-person singular subject, the auxiliary is "does".
- When we use "does", the main verb is used in its base form, "sing".
- In the second sentence, the question word "who" is the subject of the sentence.
- For that reason, we do not need an auxiliary verb.
- Instead, we can just use the verb in its third-person form, "sings".
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