"This is the book which I bought."

Translation: Αυτό είναι το βιβλίο που αγόρασα.

[Nevredimka]

Why Είναι το βιβλίο που αγόρασα is not correct?

[troll1995]

Because of this, which is emphatic. Emphasis in Greek is expressed by not omitting the αυτός/αυτή/αυτό. If the english sentence was It is the book which I bought, your tranlsation would be correct.