If one third had NO LOGIC of Basic English Grammar then it is quite safe to assume that at least 70-80 % of candidates, even if had SOME logic of grammar, made frequent Grammar mistakes. Another established fact is that around 80-85% aspirants fail the Essay or Précis paper. If we correlate these points, we see that lack of knowledge of Basic Grammar is the prime reason for failure in these exams, thus portraying Essay and Précis exams as insurmountable challenges. They are the bane of every aspirant's existence. But the fact of the matter is that mere correct grammar is enough to get you through, in these exams, unless you make some other blunder.
Correct grammar is not specific to English exams but leaves a good impression in every paper as all the exams are conducted in English (Barring literature and Isl).
Being adept at writing correct English is a MUST for clearing CSS. I have seen brilliant guys preparing for a year, focusing on optionals and working damn hard, only to fail the English Précis or Essay paper. Infact there was a guy with 789 marks in 2012 and he failed Précis' paper. Had he just passed and secured average marks in interview, he would in top-10. Importance of English can't be stressed enough. If one is not good at it, there's no shame in recognizing that as a weak link and putting in some effort to consolidate that. You guys have ample time.
Improving English demands loads and loads of writing provided your English grammar is flawless. The MOST EFFECTIVE TOOL for improving English is writing tons. Start with 1 page articles; get them checked from competent English teachers. If there are oft-repeated grammar mistakes then there's no shame in getting back to the basics, learning tenses, learning how to use punctuations, form sentences and stuff like that.
Once you have learnt, start writing again.
So it goes like:
1- Revisit English grammar. Tenses, punctuations, parts of speech stuff. It is vital that the basics be strong, only then you can build up on it.
2- Start reading some quality English daily. By reading it you will be unconsciously assimilating rich vocabulary, various sentence structures and expressions etc.
3- Start with writing one page articles and get them checked by a competent English teacher. There are no short cuts to becoming an articulate writer. There's only one rule: write, write and write more. The more you write, the more mistakes you will make while playing with different words, expressions and sentence structures. Rectification of these mistakes will ensure that you don't repeat them. Moreover by writing more, you will be making use of the entities I stated above (Vocab, expressions, sentence structure) which you read in the newspaper. By making use of these in your writing, you bring them from your unconscious to conscious memory. Once that happens, you can make use of that stuff in future at your will. That is the power of writing!
4- When your teacher gives you a go ahead that your articles are error-free, you can move on to writing expansions of about 250-300 words. Plus you can start practising other items like comprehension, précis, direct indirect etc.
Best of luck.