The correct answer is: Had I known that you wanted to read it , I would have returned it immediately. Because: "Had I known" can mean the same as "If I had known" Also: "Were you" can mean the same as "If you were" "Should you" can mean the same as "If you should" :)

Had I known_________ is the right answer. It is called literary (rich) English. See, in normal English you can say that If I had known that_____ but when you want to be rich in language ex. poems in poems such alteration of words are allowed for the sake of rhyme.

Hello my dora ,long time to find you again in italki ,your welcome , First of all you can use both the last two answers but as the last one "Had I known etc." is emphatic the correct one in Toefl terms is "If I had known...". "If I had known you had wanted to read it. I would have returned it immediately." The real situation is that you did not know that I wanted to read it. This has to be followed by the past perfect. Now if you use "Had I known you had wanted to read it, I would have returned it immediately." you would be using an inversion which is used for emphasis. The 'if' is omitted. Both are correct but then the second one - the inversion - is slightly different in that it is more emphatic and more literary. This is type 3 of conditional sentences known as "Impossible condition". The verb in the if-clause is in the past perfect tense; the verb in the main clause is in the perfect conditional. If I had known of your arrival I should have met you (but I didn't know so I didn't meet you). Here we know that the condition cannot be fulfilled because the sentence refers only to past events.

no need for the "if"because it is already understood if the sentence goes this way "Had i known that you wanted to read it_____________".

If I knew that you wanted to read it