Ellen Jovin was settling in at a collapsible table she’d set up on the Upper West Side of Manhattan last month when two little boys of around 4 and 5 approached her.

“What’s a gerund?” one of them asked.

Charmed, Ms. Jovin explained that a gerund is a verb that has become a noun after an -ing has been added to it. “Run” is a verb but “running” is a gerund, she told them.

“Are you related?” she then asked them.

One boy said “yes.” The other said “no.” Then they started fighting.

“No choking your brother at the Grammar Table!” Ms. Jovin shouted. “Oh, and ‘choking’ is a gerund.”

Ms. Jovin is a new member of a city subway ecosystem known for its string-instrument musicians, break-dancers and religion preachers. About four times a week, she schleps a foldable table and a chair to locations such as a triangular sidewalk plaza outside a subway entrance off 72nd Street or inside the underground expanses of Grand Central Station or Times Square and sets up the Grammar Table. Its laminated sign reads: “Vent! Comma crisis? Semicolonphobia? Conjunctive adverb addiction! Ask a question! Any language!”