linguisticsgirl.com: ... In my mind, grammatical *form**(what a word looks like) is clearly separate from grammatical function (what a word does). From ODO: 2.1. Any of the ways in which a word may be spelled, pronounced, or inflected 4.2. [count noun] A set order of words; a formula:
When Michael J. Fox played 'Johnny B. Goode' as Marty McFly in 'Back to the Future' in 1985, he was just aping Chuck Berry's classic fretwork for the camera. (Along with some Jimi Hendrix, Eddie Van Halen and Pete Townsend guitar moves thrown in for good measure.) Today, Fox can really play the song.
Marty McFly took Hill Valley High School by storm in 1955 when he pre-introduced its students to rock n' roll with a storming rendition of Johnny B. Goode, and 26 years later, he's still rocking the tune.