Why Teach Grammar?

In Grade 1, children start off writing one-word captions, such as dog, me, etc., for their pictures (either drawn by them or given).  By the end of the year, they are supposed to be able to write complete statements: I like dogs.
...In Grade 2, the children learn to write questions and abbreviation: Do you like Jake's dogs?
...In Grade 3, they learn to use the exclamation point: Don't hit dogs!
...In Grade 4, they learn to write quotation marks and commas: He said, "I like dogs."
...In Grade 5, they learn to write compound sentences: I like dogs, but she likes cats.
...In Grade 6, they practise these forms some more to consolidate what they have learned of the conventions of how we write.
...In Grade 7, they learn to write complex and compound-complex sentences: I like dogs that don't bite, and she likes cats that don't scratch.  Because we don't want to be hurt, we must choose pets that are safe.

However, in Grade 5 the students are expected to be able to recognize the difference between a complete sentence and a non-sentence and a run-on sentence.
...But in Grade 5, although they speak all of the patterns of sentences, they are not taught how to write them, and they sometimes write awkward sentences that they cannot tell are "wrong" by ear.  "That doesn't sound right," does not help the child who speaks a different language at home or one whose family does not speak standard English.  Furthermore, their teachers, according to the curriculum, are not supposed to teach them about the longer sentences that they can say and that they also read.

How can you tell the difference between a run-on sentence and a complex sentence?  This question was posed to me by an adult who was supposed to know the answer to this considering he had studied grammar and knew all of the jargon.  He had been to Grade 5 and Grade 7, too.  Let me tell you a story.
..In Grade 8, the teacher was teaching us grammar.  I learned all of the terms: I could locate nouns, verbs, adjectives and adverbs every time.  I remember that for about a month I found that the adverbs were jumping out at me from every page I read.  But I did not learn about the Seven Patterns of Simple English Sentences until I picked up, BY CHANCE, at the university bookstore a text that had been assigned for a course I was not taking.
...I was furious.  Why hadn't they taught us the Seven Simple Sentences in Grade 5 when I needed to know them?  How can students self-edit or edit the writings of their peers if they don't know how to recognize a simple sentence when they see one?
...To return to the question in question, you know that a complex sentence is not a run-on sentence when it is part of one of the Seven Patterns of Simple English Sentences.

High school teachers complain: They don't even know how to write a simple sentence!  Right.  That's because they don't even know how to write a Simple sentence.  Of course, the teachers are complaining that they can't write a simple essay!  And that's a topic for another blog.