I'd have thought there are a lot of people who can write any quantity of text with impeccably placed commas (and semicolons, when the required tone permits them), but would freeze and stutter and gabble if asked for "a couple of major comma rules".

I'd put myself among their number, if only because it's not clear to me what sort of thing you mean by "comma rules". Descriptions of some situations where it is appropriate to use commas? "Commas are commonly used to separate items in a list when there are more than two such items" or "commas may be used, like this, to surround interpolations -- but for all but the shortest such interpolations you should consider using parentheses or dashes, or restructuring the sentence"? Descriptions of situations where it's inappropriate to use commas? "If you have two things that can function as separate sentences, don't separate them only with a comma; use a full stop or, in some contexts, a semicolon"? Or higher-level fuzzier principles? "Stops of all kind are placed to correspond roughly with pauses in speech and boundaries of grammatical units. The comma is the 'smallest' of stops, and is generally not used where the pause would be long or where the break in grammatical structure is great." Or what?

(I suspect a cultural difference at work here: I think formal analysis of grammar and punctuation may be a bigger deal in the US than it is in the UK.)

The short answer is "pretty well." Usually I ask specifically for major comma rules, which I'd tend to define as:

* Connecting independent clauses with a conjunction * As part of a list * To offset a word or phase at the start of a sentence. * Like parentheses (or, in the lingo, an appositive phrase).

If they say, "I have no idea," that's a very bad sign; if they ask questions like yours, that's a good sign.

A fun, short book called Write Right! by Jan Venolia is worth reading if you're interested.

But that is not the same thing as "knowing the rules". A writer may be able to write perfectly correctly without being able to enunciate any rules accurately at all.

Perhaps you play one or more sports. Could you write down accurate rules telling you how to hold and move a tennis racquet, or exactly how to flex the relevant joints when kicking a soccer ball? Of course writing is more deliberative than tennis or soccer, but a good writer isn't thinking about grammar and punctuation much more than a good sports player is thinking about joint angles and muscle groups.

Also, of course, there isn't universal agreement about what is and isn't correct, nor about what the best set of rules is for describing what's correct. For instance, if you compare the famous "Comprehensive Grammar of the English Language" by Quirk and Greenbaum, and the more recent "Cambridge Grammar of the English Language" by Huddleston and Pullum, you'll find that their analyses are sometimes very different. Totally different rules, even though they're describing substantially the same language.