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The joining-comma rule mentions that two complete clauses may be joined with a comma if the clauses are short or the comma is followed by a coordinating conjunction. So, I suppose that I should write

When I arrived home I fell asleep, for I was exhausted.

When I arrived home I fell asleep because I was exhausted.

However, what puzzles me is that "for" does have a causal (subordinate) role. Here, the logic of the language escapes me. Please, can someone clarify this point?

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Remember: life is short, break the rules -- they were made to be broken. [James Dean, 1931-55] – Andrew Leach Feb 11 '13 at 23:22
Thanks Andrew, but I already break so many rules by ignorance that it would be nice to learn a bit more before breaking this one. – The Frog Feb 11 '13 at 23:38
up vote -1 down vote accepted

Well, a causal relationship can exist between two completely separate sentences, even with a sentence between them:

Alice was sent to jail. I just heard it on the news. She'd been embezzling from Bob.

So while we could join the first and third sentences as clauses in a single one, using for or because:

Alice was sent to jail, for she'd been embezzling from Bob.

It remains, that the two clauses are independent clauses.

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Are you saying that punctuating as follows is correct? "When I arrived home I fell asleep, because I was exhausted." – The Frog Feb 12 '13 at 0:47
Are you saying that you could write "Alice was sent to jail, because she'd been embezzling from Bob."? – The Frog Feb 12 '13 at 0:54
@TheFrog I certainly wouldn't worry about a comma in either of those sentences; in fact I'd positively prefer it in the "fell asleep" one. And in the "for" sentence in the question, which is the reason for my (admittedly flippant) quote from James Dean: sometimes, a "rule" needs to be broken. – Andrew Leach Feb 12 '13 at 0:56
I agree with @AndrewLeach I'd consider the comma optional in both. With, it slightly emphasises the two ideas' separate truth. Without it it slightly emphasises the causality. Both though state both the causality as well as each statement, so there's no significant change in meaning. With for there's a good argument for strongly favouring a comma before the for simply because it helps distinguish such uses from many of the other uses for has. Even there though, it can still be left out, especially with very short clauses either side. – Jon Hanna Feb 12 '13 at 1:04
Thank you very much for your answers. They are very helpful to me. As you may have guessed I am not a native speaker. – The Frog Feb 12 '13 at 8:34

Essentially, "for" does offer a reason and binds the two independent clauses together. Because the sentence is so short, a comma here would be optional. In longer sentences, it would be necessary. But I would keep the comma for clarity. And the FANBOYS usually come with the comma and two I.C.s.

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