I am a native speaker of English but am having trouble with the distinction between that and which, with respect to restrictive and non-restrictive clauses and comma usage.
For example, which is correct:
Dragons, that breath fire, are scary.
Dragons that breath fire are scary.
Dragons, which breath fire, are scary.
Since all dragons breath fire, breath fire seems to be descriptive or non-restrictive, so there should be no commas used.
My punctuation has never been that spot-on, although I don't have much trouble with other areas of grammar and syntax. I feel that punctuation heavily affects the readability of my writing even if the word-ordering of the sentence is grammatically correct. The links I've found about this have still left me slightly confused.
Edit: Upon some further reading, it seems that and which are technically interchangeable; however, that usually proceeds a restrictive clause, and which usually proceeds a non-restrictive clause. Please correct me if I'm wrong about this.