This book does not set out to be a complete grammar of the English language. Its aim, as the title implies, is to concentrate on, and to attempt to correct, the most frequent grammatical mistakes made by foreign students of English. To some extent, of course, the difficulties experienced, and the mistakes most frequently made, vary according to the nationality and the mother tongue of the students concerned, but there are quite a large number which are common to almost all, and it is with these that the present book deals. They have been suggested by those experienced in the teaching of English as a foreign language, and by foreign students themselves with whom the writer has been in touch, either personally or by correspondence.