There are a few ways to determine whether an English book bought online is authentic: 1. Check purchase history: Before purchasing a book, you can check your purchase history on Amazon or other online bookstores to confirm whether you have purchased the book before. If you have purchased the book, you can check the order information to confirm the source of the order and the publication date of the book. 2. Check the book packaging: Genuine books usually have complete packaging with clear information such as the publication date and book name. If the book's packaging was incomplete or damaged, it might be a pirated book. 3. Check the printing quality of the book: The printing quality of the genuine book is clear, the color is bright, and the text is correct. If the printing quality of the book was poor or there were typos or typographic errors, it might be a pirated book. 4. Check the book's ismn number: The ismn number of a legitimate book is the only way to determine the publication date and source of the book. If the book's IDN number is incorrect or vague, it may be a pirated book. 5. Contact the seller: If you have any questions about the quality of the book, you can contact the seller for consultation. If the seller could not provide a reasonable solution, it might be a pirated book. Please note that when purchasing books, please be sure to choose reliable online bookstores and sellers and check their reputation ratings and other information.