Explain the use of ‘a’ in the following sentence: Ayer, mi madre regaló un libro a mi hermano.

In the example sentence, the gramatical structure in question is called´the personal a’. It is used when an the Direct Object (who is being affected) is a person. In this case, the Direct Object is ‘mi hermano’, we know this because they are being directly affected by the act of the mother gifting a book (as they are the recipient). Therefore, as the Direct Object in this case is a person, we use ‘the personal a’.This is a rule that most students will not be aware of, despite having seen it in their textbooks.