There is a phenomenon in linguistics called "pied-piping," but to understand how it works, you first have to understand "wh-movement."
Wh-movement is a basic characteristic of the English language (and many other languages as well). Words beginning with wh, such as what, why, when, where, and which, are generally "pulled to the front" of a phrase or clause when they stand in place of another word in a different kind of formation. For instance, we would say "He wants it," but if we turn this into a question, the "it" becomes a "what" and moves to the front: "What does he want?" We would say, "I love him," but the "him" becomes "whom" and moves to the front of its clause in the formation "This is the man whom I love."
Pied-piping occurs when the wh-word brings another word or phrase along with it. For instance, in "This is the man to whom I owe my life," the word "to" has been dragged up front with "whom." In this case, the pied-piping is optional. You could also say, "This is the man whom [or who] I owe my life to," but that involves ending a sentence with a preposition and many people prefer not to do this for stylistic reasons. Pied-piping is always necessary in the left-branch condition -- which is way too complicated to explain, but can be illustrated with the transformation from "I like this book" to "Which book do you like?" Book has to go with which . . . no matter what. No one would say "Which do you like book? "
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