Many writers are often tempted to simply hit the button for grammar checkers on the word processing program when they finish writing a document, and call the document ready for reading. Unfortunately, grammar checkers on word processing software tend to be unreliable in many cases, sometimes going as far as to change a word that was correct in the first place to an incorrect word. Sometimes these grammar tools are sufficient to accurately check the document for errors, but in most cases, a writer will need to proofread the document even after the grammar tool has been used.
Writers who are composing a work of fiction are more likely to encounter problems with grammar checkers. Fiction writers tend to use sentence fragments, words that mimic sounds but are not actually words at all, or even words misspelled on purpose. A grammar checkers will attempt to change all these errors because they assume the conventional rules of writing are being followed. These grammar tools cannot tell the difference, in other words, between a work of fiction and a work of non-fiction; they cannot tell the difference between figurative language and literal language, and they cannot tell when the writer is using poor grammar and syntax on purpose or when he or she is making mistakes.
Some words are spelled similarly, and grammar checkers may not know which word the writer intended to use. The checker can, therefore, change the word from one that the writer meant to write into one that was not meant to be written. The checker cannot read the word in context with the rest of the sentence in some cases, meaning it will make corrections that end up being errors. The writer will need to watch the grammar check carefully to ensure these errors don't occur, and once the checker finishes its job, the writer will still need to proofread the document to look for errors.
Sometimes grammar checkers are exceptionally useful. Extremely short documents can be proofread quickly with the checker, and the reader can then read the document quickly to ensure it is ready to be read by others. The writer will still, however, need to proofread to ensure the grammar checker has not made any unintentional errors. Grammar checkers can also make layout and formatting corrections the writer had not intended; extra spaces between words will be cut down, even if the writer does not want it, and extra spaces between paragraphs may be altered as well.