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I read this sentence on an educational website:

Now times have changed and you are ready for situations involving forces in two dimensions.

Shouldn't there be a comma before and, since the subject of the sentence has changed (things vs. you)?

Now times have changed, and you are ready for situations involving forces in two dimensions.

What is the exact comma rule for joining two clauses with and?

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Duplicate? Not quite. Unlinke english.stackexchange.com/questions/412/…, this question specifically addresses clauses, not merely "items in a list" –  The English Chicken Sep 21 '11 at 22:47

1 Answer 1

up vote 5 down vote accepted

Technically, yes, a comma should be used before "and" in that sentence:

Use commas to separate independent clauses when they are joined by any of these seven coordinating conjunctions: and, but, for, or, nor, so, yet.

It's a compound sentence, as you identified, and should therefore have a comma.

That being said, a more reasonable guideline governing comma use is that they, like all punctuation, should be used to reduce or eliminate ambiguity. You can often eliminate the comma

if both independent clauses are quite short, especially if the two clauses are very closely related, and even more so if the subject of both clauses is the same, or

if only the first clause is quite short, especially if the two clauses are very closely related, and even more so if the subject of both clauses is the same.

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2  
Hmmm. I like this answer better than mine. Deleting, and +1. –  T.E.D. Sep 21 '11 at 21:30
    
All very nicely put. I think the general trend is to use less punctuation these days, and it seems to me you recognise (and delicately endorse) that trend with your "...should be used to reduce...ambiguity. You can often eliminate the comma". –  FumbleFingers Sep 21 '11 at 22:12

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