What Is a Complete Sentence?

Narrations should be done in complete sentences. Questions should be answered in complete sentences. This seems like a simple requirement, but I found myself struggling to explain to First Son exactly what makes a stream of works a complete sentence rather than an incomplete one. He would consistently answer my questions starting with something like "so that..." or "fighting..." or "loving God..." (the last is his standard answer for why someone is a saint). In the beginning, when I prompted him to respond in a complete sentence, he would often look at me in dismay.

I finally resorted to examples:

"St. George fought the dragon."
"fought the dragon"

Eventually, First Son seemed to pick up on the idea and could consistently correct his phrases when I pointed out they were incomplete.

I was delighted to discover what is likely the answer to my problem in Charlotte Mason's Towards A Philosophy of Education. I imagine this particular idea is addressed in lots of other books about Charlotte Mason's methods, but somehow I missed it.

Every sentence has two parts, (1), the thing we speak of, and (2), what we say about it.

The first part of a sentence, of course, is the subject. 

How simple it would have been to say to First Son, "Who or what are you speaking about?"

Now that I think about it, I'm sure I will be saying that to him in the future.