Questions & Answers about Mijn boek ligt er.
Why does this sentence use er to mean "there" instead of daar?
Er is an unstressed, neutral word for a location you already know about. You use it when the location itself isn't the main point of the sentence. If you wanted to point your finger and emphasize "My book is lying THERE!", you would use the stressed word daar instead.
What exactly is er replacing in this sentence?
As a location replacement, er substitutes for a preposition and a place. For example, if someone previously asked if your book was on the table (op de tafel), you can just say Mijn boek ligt er to mean "My book is lying there" without having to repeat the exact place.
Can I put er at the beginning of the sentence to say "There lies my book"?
Not when it's used as a place replacement like this. Because er is unstressed, it generally cannot be put at the front of the sentence to point out a location. If you want to start the sentence with "There...", you must use the stressed word daar (Daar ligt mijn boek).
Why use ligt (lies) instead of just saying the book is there?