A discussion of this construction comes up about once a month. Is there or is there not a comma after “that” in the following sentence.
…I knew that, if I turned right, I’d get there sooner.
The sentence is “I knew that I’d get there sooner.”
“I knew” is the subject and verb of the sentence. “That I’d get there sooner” is a dependent clause that is a noun direct object.
In the middle of the noun clause — between “that” and “I’d get there sooner” — there is an adverb clause. An adverb clause that comes in the middle of what it modifies is surrounded by commas.
I am not sure how “I knew that” can be ignored and dismissed as “an expression.” It is the subject and verb for the sentence. It is not a throwaway. Since every word in every sentence with almost NO exceptions has a function in a sentence, we have to assign these words “I knew that” a function.
The word “that” is a pronoun that can point out something (as a demonstrative) or be a relative pronoun that begins a dependent clause. There is nothing else it can do in a sentence. In this sentence, it begins a dependent clause, “that I’d get there sooner.”
If “I knew,” in fact, are the subject of the sentence, then “if I turned right” cannot possibly be introductory since it does not begin an independent clause.
I know this rule from Gregg. I just don’t think there is any way that it matches the grammar of the sentence. You cannot just dismiss words in a sentence and call them “an expression.”
I would so like the authorities to back up their rules with a discussion of the grammar. I believe that all rules have to go along with the way English grammar works.
And with this explanation, I will let it go — until this comes up again. smile emoticon