One of the reasons I appreciate Sentenceworks is because it teaches the rules of the English language rather than just pointing out the errors and having a computer automatically fix it. If you’ve been using Sentenceworks regularly since the beginning of the school year, by now you should have learned to correct at least three or four major problems without the use of a grammar checker. (If you’ve learned to correct more than that, we’re very proud of you. 
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Learning to follow the rules is important; learning to break them is just as important because it gives purpose to following those rules. Breaking the rules is not just a random act of disobedience, but a means of broadening one’s learning. For instance, a baby will often fall down when it’s learning to walk. In falling, the baby will learn to avoid falling, perhaps how to fall properly so that it doesn’t get hurt, and also how to bend its knees, how to use its arms for protection, and how to roll. Perhaps the baby also has an older sibling, and learning to fall on purpose will teach the baby how to manipulate the parent. (Billy, did you push your little sister again? Into the Thinking Chair you go!)
The same concepts can be applied to English rules. If I ! deliberately ,use punctuation incorrectly? it will only serve to make “a” point My readers :will spend a lot of time ‘focusing on how vitally\ important punctuation, is to writing.
On Language; Breaking the Rules is an old-but-good article on how some of the rules came to be. By understanding the origins of the rules, a writer can understand when the rules must be followed, and when they can/should be broken.
For those who are truly enthralled by such exploration, there is an excellent book called Miss Thistlebottom’s Hobgoblins which explores the rules in depth.
Formal writing is not just about following grammar rules. It’s about using the elements of a language to effectively communicate your thoughts and ideas; the more weapons you have at your disposal, the greater the attack. Have fun building your arsenal!