"In theory, a computer grammar checker should work like a spell checker. It should identify errors and suggest alternatives for corrected text. However, spelling is fairly straightforward. In comparison, grammar is more much interpretive. Despite recent advances in computer technology, it is very difficult for a machine to learn to properly understand the English language. In written English, there are a number of expressions that have multiple meanings and the context of a word's usage can directly affect the tone of a sentence. These subtleties are simply impossible for a machine to fully comprehend, thus leaving grammar checkers with three main flaws: A grammar checker may miss important errors. A grammar checker may accidentally suggest corrections that are inaccurate. A grammar checker may flag grammatically correct text as incorrect. There are many common grammar errors that computer software struggles to find. For example, one recent study comparing the effectiveness of common grammar checkers found that these programs will typically miss the following errors: No comma after an introductory element in a sentence Missing preposition Comma splice No comma in a compound sentence Vague pronoun references Tense shift Incorrect use of the possessive apostrophe Pronoun agreement error Run-on sentence Sentence fragment If you're truly interested in learning to improve your grammar, it's best not to rely on software to catch your errors. Instead, brush up on basic grammar rules by visiting the following websites: Daily Grammar http://www.dailygrammar.com/ Grammar Monster http://www.grammar-monster.com/index.html 1 Language http://www.1-language.com/englishcourse/index.htm English Page http://www.englishpage.com/ Superteach http://webnz.co.nz/checkers/grammar2.html " Source and further information: http://www.yourdictionary.com/dictionary-articles/free-online-grammar.html