Improving English language is imperative for success in CSS. Not only that the very initial couple of papers – English Essay and Précis & Composition – filter out most of the candidates not well versed with English writing skills, the candidates are also required to attempt all papers in English language. Having a good English grammatical foundation is indispensable in this regard.
The best way to improve understanding of English grammar is to study and acquaint oneself with rules of English grammar. A very useful book to learn basic rules of English grammar is ‘English Grammar in Use’ by Raymond Murphy. It contains grammar rules with exercises to help you find out and correct your mistakes.