In my opinion, understanding dependent clauses and the way they work inside a sentence and how they are punctuated is at the very heart of understanding the language. When clauses are punctuated correctly, it helps the reader decipher what is going on in a sentence and produces a sentence that flows and is easy to read.
With that said, I would point out that clauses can get complicated. There are clauses within clauses that are within clauses. However, the basic punctuation does not change even when the clauses are buried inside each other. Sorting out the types – there are three: noun clauses, adjective clauses, and adverb clauses – and knowing which one you are dealing with and how that one is punctuated is the key.
An adverb clause at the end of what it modifies is right where it belongs and generally takes no punctuation. Pulled out to the front, it takes a comma. When an adverb is in the middle of what it modifies, it is surrounded by punctuation.
[The parentheses here are added simply to point out the clause and are not being used to punctuate the sentence.]
…I will help you with your computer problem (when you get here).
…(When you get here), I will help you with your computer problem.
…I will help you, (when you get here), with your computer problem.
…He told me about the incident on the train (after I arrived.)
…(After I arrived), he told me about the incident on the train.
…He told me, (after I arrived), about the incident on the train.
These are the basics of punctuating adverb clauses. These commas are not optional and do not depend on the “flow” of the sentence. This is the way the language works.
The rule does not change when the adverb clause is a part of another clause.
[The brackets and parentheses are added simply to point out the clauses. They are not being used to punctuate the sentence.]
…He resigned [because he would have been demoted (if he had stayed)].
Both the “because” clause and the “if” clause are at the end of what they modify and need no punctuation.
…He resigned [because, (if he had stayed), he would have been demoted].
Now the “if” clause is in the middle of the “because” clause and needs to be surrounded with commas.
…I have a friend [who, (when he turned 50), decided to go back to school].
…[Whether you, (after the semester is over), want to continue with the program] is your choice.
In each case, the adverb clause is in the middle of the other clause and needs to be surrounded with commas.
So let’s address a question that recurs and for which there are varying ideas about punctuation.
…I know (that he will be here to help me).
…I know (he will be here to help me).
“That…help” is a noun clause that serves as the direct object for “that” and does not take any punctuation. Because it is not doing anything grammatically within the clause, the word “that” could be left out, which is going to become a problem when the construction gets more complicated.
When there is an adverb clause in the middle of the “that” clause, it takes commas around it as all adverb clauses do when they are in the middle of another clause.
…I know [that, (if I call him), he will be here to help me].
…Do you remember [that, (when I saw you at Christmas) you promised to help me with that]?
…She said [that, (because she had had pneumonia), her lungs were problematic].
This construction gets dicey when the word “that” is not there or is there twice or is on the “wrong” side. It is just bad grammar and simply does not matter. The adverb clause is in the middle of the noun clause and needs to be surrounded by commas.
…He thinks that, when he comes over, he can do anything he wants.
…He thinks that, when he comes over, that he can do anything he wants.
…He thinks, when he comes over, that he can do anything he wants.
…He thinks , when he comes over, he can do anything he wants.
Disclaimer: There are authorities who believe that only one comma after the clause is needed.
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