Mulroy is a now-retired professor of Classics who found the biggest obstacle to teaching undergrads Latin or Greek was that they knew nothing of English grammar.
He's certainly big on knowing parts of speech. He also strongly recommends good old-fashioned sentence diagramming to learn how languages work.
I recently wrote a blog post about this -- Does It Matter if Students Can't Identify a Verb? http://mayathiagarajan.blogspot.sg/2016/03/does-it-matter-if-kids-cant-identify.html