In sixth grade, presumably to prepare us for learning from multiple teachers, we spent three days a week learning old school grammar from an old school teacher. Nouns weren't only "people, places and things," they also included "ideas, qualities and feelings." We took a massively intimidating test four times that year. Half of this test involved throwing all the grammar rules we'd learned onto a copy of the exact same test sheet the teacher had used for over 40 years. As in, photocopied ditto pages that had years of reproduced smudges, folds and staples. The other half was sentence diagramming. Ms. Riggs claimed no student could be tricked by any grammar question, so long as they understood her lessons. Grammar was easy.
She wrote "the easiest sentence in existence" on the overhead: "Go!"
Before any of us had a chance to steal her thunder, she diagrammed the sentence on the overhead.
"The thing is, this sentence has an implied subject; none of you know about that yet, because I haven't taught you."
(you) | go
Ms. Riggs didn't teach us much about the real world, but she did show us that grammar and language (even English!) generally have a pattern. These patterns can be seen anywhere we use language.
Just as our minds imply the subject "you," our minds generally imply a bright orange sign reading LEFT LANE CLOSED AHEAD means a driver should pay attention, because the left lane will be unavailable soon.
Our minds generally imply a bright orange sign reading BUMP means a driver should be wary of an upcoming bump in the road.
Our minds generally imply a bright orange sign reading ROAD WORK means the road is being worked on ahead.
Each of these construction signs are informational, offering a heads-up as to what to expect so drivers can, ideally, plan ahead based on the new, likely revised, traffic patterns ahead.
None of these signs give a command.
So what information does the sign SLOW offer?
SLOW (machines in the way)?
SLOW(ly making progress)?
(You're going too) SLOW(, speed up)?
None of the other signs have given a command; why should we assume SLOW has?
So bless this young hoodlum's vandal heart:
(YOU) SLOW DOWN is a command and clear; SLOW is informational and equivocal.
SLOW DOWN offers a beautiful example of an implied subject.
(YOU) | SLOW
From the tall neighborhood dude who walks and bicycles everywhere to the un/sub/conscious word warrior with a spray can: Thank you!