Practical English Usage Third EditionThe purpose of this book English, like all languages, is full of problems for the foreign learner. Some of these points are easy to explain for instance, the formation of questions, the difference between since and for, the meaning of after all. Other problems are more tricky, and cause difficulty even for advanced students and teachers. How exactly is the present perfect used? When do we use past tenses to be polite? What are the differences between