I have trouble deciding when to put a comma before so. The following are two examples about so. But one with a comma while the other not. Please explain when we should put a comma and when we should not.

She asked me to go, so I went.

The shops were closed so I couldn't buy anything.

Thanks very much.

Truthfully, you'll get a different answer based on opinions because there is no hard and fast rule for something like that (that EVERYONE will agree on). The Harbrace handbook explains that "so" should be thought of as a conjunction. You always put a comma before so, unless "that" follows "so." In other words, I am writing this so that you will understand when to use a comma...see, that sentence doesn't get a comma.

I would use a comma in both of your examples. Why? Well, without a comma in the second sentence, it doesn't literally say what the author intended it to. It's really saying that the shops were closed because you couldn't buy anything rather than what it wants to say: Because the shops were closed, I couldn't buy anything.

My other reason for putting a comma in both: both of the sentences are compound sentences. As a rule, all compound sentences must be separated with a comma and a conjunction. In this case, "so" is the conjunction. There is a growing trend to remove the comma before the and in such cases, but I disagree with it, and I will fight to the death to keep it there. People who take it out are just lazy writers, plain and simple. (Of course, there is a push from certain media outlets and publishers to remove commas to save ink and space...but that is really only degrading the English language and the way things should be.)

Others would disagree with me, simply because they hate commas. I tend to use commas more than anyone I know, but I always have a reason for putting a comma, whereas many people can't tell why they would remove it. Usually I'm told: well, I didn't like the comma there.
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I agree that both of those sentences would benefit from a comma, but I must take issue with some of your reasoning, “anonymous”. That second sentence could never be construed to read "the shops were closed because you couldn't buy anything", nor does it "literally" mean that. I challenge you to find one person who would ever read that as the intent of the sentence - and I guarantee you won't be able to. “So” and “because” are simply two words that have totally different meanings in the English language, comma or no comma.

<stands on soapbox>The only thing worse than a "lazy writer" is a lazy pedant. And that is exactly what I am calling you out as. To say that the omission of a well-placed comma is "degrading [to] the English language and the way things should be." (emphasis added) is to give no reasoning at all. Who appointed you All-Knowing Prescriptivist anyways?</stands on soapbox>
Place a comma before so if the proceeding clause is independent (nonesential):

--independent clause--

It rained, so we could not play baseball.

...we could not play baseball is an independent clause; that is, it can stand alone as a complete sentence; therefore, a comma should precede so. Some writers may omit the comma, however, given such as short sentence, but for lengthier ones, a comma is a must.

DO NOT place a comma before so (or any other of the seven coordinating conjuctions) if the following clause is dependent (essential):

-------dependent clause---------------------

He disguised his voice cleverly so that we couldn't identify him as the caller.

...that we couldn't identify him as the caller is a dependent clause; that is, it cannot stand alone as a complete sentence; therefore, a comma must not precede so.
"The shops were closed so I couldn't buy anything."
=> The shops were closed to prevent me from buying anything.
(Clarifying example: The car stopped so (that) I could cross the street.)
"The shops were closed, so I couldn't buy anything."
=> I couldn't buy anything because the shops were closed.
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I agree that a comma belongs in both sentences. However, the sentence definitely does not mean that 'the shops were closed because you could not buy anything' and could not be construed as such. There is an inherent problem with the sentence, and without the comma it implies that the stores closed to prevent the buyer from buying anything. I think this may be what anonymous was trying to get at. Using the comma clarifies this and allows the reader to truly understand the meaning of the sentence. That is: 'I could not buy anything because the stores were closed.'
Hi. Please tell me if we could put a comma with a sentence with the word "so" (when it functions as the phrase "so that" and essential) when the part before it is long and perhaps has a coordinating conjunction like "and"?

How about this as a reason to put a comma before a sentence like the one I tried point out above?

If one has to pause after a long part of the sentence before the word "so," will it be a good enough reason to put a comma eventhough what comes after "so" is essential? I hope what I wrote reflects what I wanted to say - not sure though.

Also, I am sorry I can't give an example sentence. Any help will be appreciated.
Certainly not. When the word so introduces a purpose clause, for example "I went home so I could watch it on TV" or modifies a modifier, for example "It was so heavy I couldn't lift the rock" it does not take a comma. Use the comma to separate independent clauses, for example "I was home, so I watched in on TV" and "The rock was big, so I couldn't lift it."

Read more: http://wiki.answers.com/Q/Do_you_always_need_to_put_a_comma_before_the_word_so#ixzz1W4wm77ve
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She asked me to go, so I went. = I went because she asked me to go.

"The shops were closed so I couldn't buy anything." That wouldn't mean, "The shops were closed because I couldn't buy anything." It would mean, "I couldn't buy anything because the shops were closed."

Your challenge stands. A little odd, but it stands nonetheless.
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