commas with adverb clauses

(A similar topic is discussed in French in the article VIRGULE AVEC UN COMPLÉMENT DE PHRASE.)

The use of commas with an adverb clause depends on the position of the clause and its degree of importance.

Adverb clause at the beginning

When placed before the independent clause, an adverb clause takes a comma after it.

• When I hear the song "Spanish Eyes," I always feel like dancing.

Note: It is possible to omit the comma if the clause is quite short and does not require a pause:

• When you call please mention my name.

But even with a short clause, make sure to include a comma if there is a risk of misreading:

• When you call, Janice may be out.
  [comma prevents misreading When you call Janice]

Adverb clause in the middle

When placed in the middle of the independent clause, an adverb clause generally acts as an interrupter and requires a pair of commas (one before and one after).

• My cousin, whenever she travels, sends me a postcard from every port.

Adverb clause at the end

When placed at the end of the independent clause, an adverb clause may or may not require a comma, depending on whether it is essential to the meaning of the sentence.

Essential clauses provide essential information and are not set off with commas.

• The package came after you had left.
  [identifies the time of delivery]
• The police will press charges if the witness’s statement is true.
  [stipulates the condition governing the action in the main clause]

Note that clauses introduced by until and the conjunctions of comparison than and as … as are normally essential:

• They took shelter in the cave until the storm ended.
• Jorge is taller than Greg (is).
• No one can play that sonata as well as Marsha (does).

Non-essential clauses provide additional, non-essential information and need commas:

• The package came at 8:30, after you had left.
  [The phrase at 8:30 identifies the time of delivery; the clause is just additional information.]
• The passenger apparently arrived late, if the witness’s statement is true.
  [The adverb clause if the witness’s statement is true has no bearing on the passenger’s arrival time; it is just an added comment that is not essential.]

Note that clauses beginning with although, even though, though and whereas are normally non-essential:

• Lisa won the prize, even though (although, though) the competition was stiff.
• Cara wanted to paint the room blue, whereas Paul preferred green.

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