Writing good English is still an important part of creating a good novel. Of course, this statement should be almost as obvious as the fact that cheddar is yellow and that finding an agent is like looking for an invisible needle in a very large haystack. There are three aspects to writing good English: spelling, grammar and style.
Spelling is making sure that your words are spelt correctly, i.e. in the acceptable way. Assuming you use Microsoft Word, make sure that automatic spell checking is turned on (you know, the red squiggles.) However, there is something you should be aware of. Word does not do contextual spell checking, so for instance if you spell “I always lose at chess” as “I always loose at chess,” then Word won’t detect the mistake. So there’s no excuse for not reading carefully through the manuscript to make sure that spelling mistakes are found. It’s always a good idea to give the manuscript to someone else to proofread before you submit it, as the eye can become so accustomed to words that it sees what it wants to see. This has happened to me on many occasions.
Next, comes grammar. Correct grammar means that your writing obeys the rules of the English language. Entire books and lives have been devoted to this subject so I won’t dwell over it. The best thing to do is to get a pocket-sized grammar book and to dip into it occasionally. Knowing the precise rules of English is vital for an author.
Finally, comes the killer: style. Style is to writing what hospitality is to a hotel. Even if your spelling and grammar is perfect, it won’t satisfy the reader if your writing lacks style. Style is all about knowing how to use words and sentences in such a way that the very writing itself is pleasing to the reader, which might actually mean breaking the odd grammar rule on occasion (after all, one of the beauties of English is that it is a very flexible language). The de facto standard book on this subject is “The Elements of Style” by Strunk and White.