If you are using a self-study book like English in Use, try doing the exercise before you read the grammar explanation. When you have read the explanation, see if there are any answers you would like to change. Then check your answers with the key at the back of the book, but don't mark or change any of the answers on the page. One or two weeks later, go back and look at your answers again and see whether you can remember which ones were wrong and why.