I am having trouble figuring out when to use commas to set off "nonessential" information. Sometimes it's obvious:
Bob, who is thirty years old, is an alcoholic.
But other times I'm not sure:
The day he quits drinking * he will start a llama farm
He has his heart set on owning El Duderino ranch * in New Mexico.
In the first case, my ear says that there should be a comma at *, even though information before it seems essential to me. In the second case, the stuff after * is not essential, and yet it seems a little much to use a comma.
Even that last sentence I wrote confuses me. "In the second case" seems essential but I used a comma. Is this correct?
Does it depend on personal style and the length of the clause? Or perhaps I'm misinterpreting the meaning of "essential" in this context?