When to Trust Your Grammar Checker
The answer? Almost never.
Grammar checkers are not usually a good measure of a sentence's accuracy. Not only will they generate false positives and tell you a perfectly correct sentence has errors, but they are also perfectly willing to let huge errors slip by. As a general rule, you're likely better off turning the grammar checker off and just double-checking your text yourself—or better yet, asking a friend or colleague to. Often our brains will automatically correct simple mistakes in text, replacing them with what it expects to be there, so a second set of eyes is always useful.
Remember, a grammar checker is trying to parse human language and determine if it is acceptable or not (and offer possible solutions to fix it!) The fact of the matter is, doing this correctly and consistently is beyond the current capabilities of software. The field of technology concerned with computers' ability to interpret language, including grammar checkers, is called Natural Language Processing.
When it comes down to it, the grammar checker doesn't actually know any English; it's just applying a set of mechanical rules to human problems. And though you can add words to a spell checker, a grammar checker can't learn new things (unlike you!) So here are some of the pitfalls you can encounter when you trust your grammar checker.
I. False Positives
The most common grammar checker mistake that confuses learners is when a false positive is generated; that is, the grammar checker flags your sentence as incorrect even though there is nothing wrong with it. Here is an examples of a false positives that a grammar checkers gave one of our users:
"We were also innocent people, but who helped us?"
Grammar checker error: This clause may not be clearly phrased as a question, despite the use of a question mark. Consider putting the auxiliary verb first, or using a question tag.
As we determined in the question asked, there is nothing actually wrong with that sentence. It's perfectly fine, but the grammar checker was confused. In fact, the more proficient you become in a language, the more likely you are to get false positives from your grammar checker. New learners tend to stick to simpler sentence structures, but as our knowledge of a language grows we branch out into more complex sentences, and learn which rules can be bent or even broken for artistic effect. The more complex the sentence, the more likely a grammar checker is to misunderstand and flag it.
Instead of relying on a grammar checker to tell you what's wrong with a sentence, use your own judgement. Try to look up examples of similar sentences, and ask yourself if the sentence sounds right. If you think there's something wrong with a sentence, or simply aren't sure, go ahead and ask us a question about it. But don't trust your grammar checker over your own knowledge and instincts; more often you're going to be right.
II. Missed Mistakes
Even more dangerous than the false positive is when your grammar checker silently lets mistakes slip by. We don't get questions about these due to their very nature—the grammar checker hasn't alerted you to it, so you don't even know anything is wrong! But it's very important to be aware that this does happen, and probably more often than you think. Here are some examples of incorrect sentences that grammar checkers won't bat an eye at:
They returned to quickly.
The boy the girls love are here.
You are going to want you're project actively managed.
These are the correct versions of these sentences, though your grammar checker won't tell you so:
They returned too quickly.
The boy the girls love is here.
You are going to want your project actively managed.
The University of Washington's website contains this letter, written by an EFL teacher to show the shortcomings of spelling and grammar checkers. Her grammar checker found no problems with this email. Can you spot a couple dozen?
Deer stew dents
To questions four you. Do you know how too use the spell checker on
Can you sea sum spelling mistakes inn this?
The spellchecker on my computer could knot fine any problems - awl my
words were correct. The grammar checker all so said my grandma was
III. Parting Advice
In short, grammar checkers are a mixed bag. They'll often miss your mistakes or get you wondering what's wrong with a sentence that's perfectly fine, though they can catch some of the simpler missteps (there vs their vs they're, for example). If you do use them, take their advice with a grain of salt. Better yet, turn the grammar checker off and go with your instincts instead. Use what you know, do research, have someone else double check your work, and if you find a sentence particularly perplexing give us a ring. We're here to help you sort out anything you're unsure about. But don't ask us why your grammar checker tells you something's wrong with that sentence; we'll tell you Because you turned it on!
Here are some other questions that are related to this subject and which you might find useful: