I've been perplexed by a certain usage of the uninflected verb, and see it quite often. For example, I happened upon the following passage in reading on the pre-war South: "If a white person assemble with Negroes for the purpose of instructing them to read or write, he shall be confined to jail not exceeding six months and fined not exceeding one hundred dollars" –– Virginia. Code , 747-48 In the first sentence, why is the verb "assemble" unconjugated or invariant? Rather than, "If a white person assembles with... Or in a second example, which I pulled off a sports forum today (at realgm.com about a Knick basketball player, if you're curious): No shirtless pics, I am disappoint. Why is it not as follows?: "No shirtless pics, I am disappointed." What is the specific grammatical rule (or rules) governing this?