commas with absolute phrases

One form of non-restrictive expression is the absolute phrase: a phrase grammatically unconnected with the rest of the sentence. An absolute phrase consists of a noun or pronoun followed usually by a present or past participle modifying the noun or pronoun.

Absolute phrases are followed by a comma:

• Weather permitting, the conference will be held as planned.
• The chapter completed, I returned to my former duties.

Note the following errors in the punctuation of absolute phrases:

• The investigation had been completed, and the results, having been known for some time, the public was anxiously waiting for heads to roll.
  (Correction: remove the comma after “results.”)
• We were unable to answer her questions. The truth being that we hadn’t given the matter much thought.
  (Correction: replace the period after “questions” with a comma or dash.)

Absolute phrase vs. dangling participle

Do not confuse absolute phrases (which are grammatically correct) with those involving dangling or unrelated participles (which are errors). In an absolute phrase, the participle modifies the noun or pronoun coming before it, within its own phrase.

But in the case of a dangling participle, the participle must modify a noun or pronoun in the rest of the sentence; and it dangles when it does not have the right word to attach to, as in the following examples:

• Listening to his speech, it felt as if he would drone on all day. (Problem: The subject “it” was not listening to the speech.)
• Uprooted by the gale-force winds, the garage roof was damaged by the falling pine. (Problem: The subject “roof” was not uprooted by the winds.)

This common problem is avoided if the sentence is recast so that the subject of the independent clause is the same as the person or thing performing the action in the present participle (or receiving the action in the past participle):

• Listening to his speech, I had the impression that he would drone on all day. (Correct: The subject “I” was listening to the speech.)
• Uprooted by the gale-force winds, the falling pine damaged the garage roof. (Correct: The subject “pine” was uprooted by the winds.)