Grammar is sooooo slippery.
If anyone has advice on teaching the distinction between restrictive and nonrestrictive modifiers, I would love to hear.
Here's my latest stab at it. (Katharine told me about the "Pause Test.")
And here's the boyfriend who plays the piano, an example I learned from a graduate student and Fulbright scholar whose native language is Arabic.
She was the graduate assistant in an Applied Grammar course for English majors I was auditing, and she was gobsmacked by how little grammar American students know. One day she said (paraphrasing), "The reason to learn grammar is to appreciate your language. We study grammar because Arabic is a beautiful language."