Grammar is one of the most difficult of concepts for the average writer to master. Even the most fluent of authors, one whose words soar like Condors over the Andes, may fervently need the aid of one, two, three editors. This is because, unlike mathematics, grammar is an art and not a science. While many grammarians and pedants will scrawl many words of rebuke to all the miscreants who abuse grammar daily the fact is that even grammarians have much they cannot agree on.
Consequently the world has been blessed with the spell check and the grammar check in software programs such as Microsoft Word. I'm the first to tell you that I'm a big fan. I'm one of those writers, of which there are many, who has come slow to understanding the uses of grammar. I have read several books on the topic and taken many college courses that have dealt with it on some level or other, but like the concept of God, much of it I have to take on faith and go with my gut, or rely on my Word program.
Lo, and Behold, I am here to tell you not to rely on your Word program. It is no substitute for true knowledge or the sharp red slash of an editor's pen. I have a tendency towards being far too slap dash. I'm always in a hurry. I gulp down food with the same avidity with which I dash out my lines of deathless prose. Many is the time I have happily submitted my words to the light of electronic publication, hit the post button, satisfied by a clean bill of health from my grammar and spell check only to read again later and have my self-satisfied smile torn from my smug lips.
The most common error of a writer relying on the tools built into their writing software program is reliance on the spell check. Spell check cannot read your mind or know your intentions so it's quite common for it not to catch homonyms. There are a number of classes of homonyms from words that sound the same but are spelled differently to ones that are spelled the same and have different definitions, to ones that are similar in sound but not identical and so forth. Many homonyms can render a sentence not only nonsense but often create a humorous effect that you simply don't need such as substituting the word 'waste' for 'waist' in an erotic passage.
Coming back around to grammar the computer is even less of an authority. It can only be an aid, not a substitute for knowledge. Many is the times that my Word document will suggest that I change something when, in fact, I was correct but a little more complex than the software is programmed for. Unless I know grammar rules I cannot choose the right option: change or ignore. At other times the software generated suggestion is not the correct one, but my grammar is still wrong. What do I do? And then it is simply true that as a creative writer I might want to break the rules. For it to be art and not accident I need to know the rule first - and then disregard it as an artistic choice.
In conclusion I'm here to urge you that if you love words and desire to be a writer of any competence that you take to heart the study of words, their meanings and corrects spelling, and in addition attempt to tackle the mysterious art of grammar. You will never be perfect, not even the most erudite and astute among us is, but these are the tools of your craft. If you were a painter you would not eschew the easel, brush, and palette without at least first having excellent knowledge in their use. And then once you have competently crafted your piece submit it to the scrutiny of others because, unlike in painting, spell errors, typos, and bad grammar are pernicious beasts that like to skulk and hide to trip up self-satisfied writers.