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I am trying to teach my students about commas and one of them asked if generally, not all the time, a comma comes before the full subject. I believe this is 90% of the time the case. I was wondering what reason I can give. Is it part of some rule. Examples...

While I was eating, the cat scratched at the door. Because her alarm clock was broken, she was late for class. If you are ill, you ought to see a doctor. Having finished the test, he left the room. To get a seat, you'd better come early. However, you may not be satisfied with the results. Finally, I went home.

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I'm not sure I know what you mean. Most sentences have no commas. Can you give some examples? – nohat Aug 16 '12 at 4:41

It may do, but that is incidental. In each of your examples, the comma is used to set off a weak interruption to the sentence. Commas are frequently used for this purpose in pairs as bracketing commas, but as Larry Trask points out in his ‘Guide to Punctuation’, ‘Sometimes a weak interruption comes at the beginning or at the end of its sentence. In such a case, one of the two bracketing commas would logically fall at the beginning or the end of the sentence — but we never write a comma at the beginning or at the end of a sentence.’

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The purpose of the weak interruption mentioned by Barrie is pragmatic. We need help in reading and understanding more complex sentences. In print, a comma is used to signal a logical shift from one thought to a related one, where a more forceful punctuation mark or a new sentence is deemed too - er - punctuative. In spoken English, pause and intonation is (/are) used.

If a comma were not used in your third and fourth examples, there would probably not be a problem, so I'd say it was optional there. In your second and fifth examples, it's starting to get a bit unwieldy without the comma. The first example definitely needs the comma to defeat the garden-path nature of the sentence :

While I was eating the cat scratched ...

Sentence connectors like however, on the other hand, in addition, furthermore, moreover, additionally, besides, firstly, secondly, next and finally always take a comma. Note, however, the difference between

Finally, we come to the question of what to do about persistent re-offenders.

where finally is used as a sentence connector, and

He finally left the office. / Finally(,) he left the office. / Finally(,) I went home.

where finally now is a matrix adverb, and the comma is not mandatory.

Other pragmatic markers, like modal and other sentence adverbs

(Seemingly, he doesn't earn enough to be able to afford a new Jag.

Politically, it becomes quite a problem.

Confidentially, you need to watch him.)

are also set off by commas.

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