"The book is so interesting that I had read it three times before I realized it"

"The book is so interesting that I had read it three times until I realized it"

Which sentence is right? Why?
Neither sentence sounds natural. The "it" at the end sounds as if it refers to the book (as it would in "this book is so interesting that I had read it three times before I lost it."), rather than "the fact that I had read it three times." I think what you want is something like "This book is so interesting that before I realized what I was doing, I had read it three times."

Definitely not "until."
Hi Nevermore1999

'Before' would be the word to use. The word 'before' simply places 'read the book three times' at an earlier time than 'realized'.

The word 'until' would be better used in reference to duration in time. This is the way you could use 'until' to talk about the same situation:

- I did not realize I had already read the book until I had finished reading it for the third time.

In other words, the duration of 'did not realize' lasted through the second and third readings of the book, and ended at a point in time after the book had been read for the third time.

(I also agree with Khoff that your original sentence is not worded very well.)
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. . . before I realized it

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"Before or Until"

It may help you to think of until as meaning "continuously until", "without interruption until".

If you can add "continuously" or "without interruption" and your sentence still makes sense, you can use "until". If there is any action which finishes during the time period you are considering, you can't use "until".

In the sentence you cited, you can't use "until". You don't read a book three times continuously. There is an interruption each time you finish reading it. You have to finish reading it once, then twice, then three times. So, of the two choices, you need "before".