Practical English Usage is not a complete guide to the English language. As the title suggests, its purpose is practical: to give learners and their teachers the most important information they need in order to deal with common language problems. Within this framework, the explanations are as complete and accurate as I can make them. However, it is not always helpful or possible in a book of this kind to deal with all the details of a complex structural point; so readers may well find occasional exceptions to some of the grammatical rules given here. Equally, the book does not aim to replace a dictionary. While it gives information about common problems with the use of a number of words, it does not attempt to describe other meanings or uses of the words beside those points that are selected for attention. Nor does it attempt to cover all the vocabulary problems that learners may meet: for this, another complete book would be needed.