Should you use a comma after a group of words introducing
the main idea of a sentence? Here are five defining sentences:
- On Tuesday he came to see me.
If the introductory words are short and are simple and contain
no verb form, do not separate them from the main idea--the independent
clause--that follows.
- When it was Tuesday, he came to see me.
If the introductory group of words has a subject and verb
but cannot stand alone as sentence, use a comma. Such clauses
generally start with when, while, because, as, since, if, although,
though, before, or after.
- Remembering me, he came to see me.
If the introductory group of words contains a word ending
in "-ed" or "-ing" and is not acting as the
subject of the independent clause, use a comma. Such introductory
groups of words are called participial phrases.
- After many hours of thought and hours of fruitless fretting,
he came to see me.
If the introductory group of words is substantial, use a comma.
What is substantial? Four words or more.
- Inside, the diners were enjoying their meals.
If a comma would prevent misreading, use one.
So the rule could roughly be reduced to this: if the introductory
element is long or has a verb form in it or if a comma would
prevent misreading, use a comma to separate the element from
the main idea which follows.
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