Quick grammar question (commas)

Test Preparation
1

I am studying for the ACT right now and stumped upon some confusion (page 52 of Princeton Review’s 2015 edition).

The book says there should be only four situations in which one should use commas:

  1. "Stop": comes between two complete ideas separated by coordination conjunctions (FANBOYS)
  2. "Go": Link a complete idea to an incomplete idea
  3. Lists and 4. Unnecessary info

Isn’t #1 just an application of #2?

A complete idea + a coordination conjunction = an incomplete idea.

So for example:
“I threw a snowball, but it hit someone.” would require a comma according to rule #1 but how is that different from rule #2? “But it hit someone” is an incomplete idea. Is there something I am missing or is rule #1 merely an example of rule #2?

And second question: how do I know I know wether or not a conjunction turns a complete idea to an incomplete idea?

“I did not throw a snowball, even though I wanted to” requires a comma, but “I did not throw a snowball, however I wanted to” can not have a comma. I can tell by the ear, but what specific rule decides that? ‘Even though’ and ‘However’ are both subordinating conjunctions, so what differentiates the one from the other and determines wether or not a comma is to be used?

2

Google coordinating conjunctions and subordinating conjunctions and that might help clarify some of your confusion.

3

http://www.towson.edu/ows/conjunctions.htm