Position of the subject is after the first conjugated verb

  Ils peuvent faire ce qu' ils veulent. => Peuvent-ils faire ce qu' ils veulent?
  They can do, what they want. => Can they do, what they want?  
  Ils ont pu faire ce qu' ils voulaient. => Ont-ils pu faire ce qu' ils voulaient?
  Could they do, what they wanted? => Could they do, what they wanted?  
  Ils ont lu un livre. => Ont-ils lu un livre?
  They have read a book. => Have they read a book?  

The subject is in inverted clauses after the first conjugated verb. If there are other pronouns in the clause, everythings stays as described in the chapter Pronouns. They are before the first conjugated verb, but in any case before the infinitive.

  Ils le comprennent. => Le comprennent - ils?
  They understand it. => Do they understand it?  
  Ils peuvent le comprendre. => Peuvent-ils le comprendre?
  They can understand it. => Can they understand it?  
  Ils l' ont compris. => L' ont-ils compris?
  They have understood it. => Have they understood it?  
  Ils ont pu le comprendre. => Ont-ils pu le comprendre?
  They were able to understand it. . => Were they able to understand it?  

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