"So" is a coordinating conjunction, so a comma is appropriate.
Which is better, a comma or semicolon for the following sentences?
I knew it was gonna be a hot day, so I decided not to go.
I'm not feeling well today, so can you come some other day?
(Not a Teacher)
I think of a semicolon as a way of connecting 2 separate but closely related statements without breaking the 'flow' of speech or thought. So there is a bit of artistic license involved in their use. However, you are soliciting our opinions, so here is mine:
"Both sentences are perfectly fine the way they are; I don't think a semicolon would make any improvement."
***** NOT A TEACHER *****
I agree with the other posters that in 2014, the word "so" is considered to be a conjunction. Therefore, a comma is fine.
In reading "older" English, however, please do not be astonished if you see the writer use a semicolon or a period (full stop).
1. Some older authorities say that "so" should be considered an (transitional) adverb.
2. Thus, they would probably feel that you should punctuate your sentence like:
a. I knew that it was going to be a a hot day, and so / hence / therefore / consequently I decided not to go.
b. I knew that it was going to be a hot day; so I decided not to go.
c. I knew that it was going to be a hot day. So I decided not to go.
As you can see, sentences a - c are very elegantly punctuated -- in my opinion.
P.S. If you are doing some FORMAL writing, it is only my OPINION that punctuating your sentences like a - c would be a good idea.
Source: Pence and Emery, A Grammar of Present-Day English (1963).
Last edited by TheParser; 17-Aug-2014 at 16:54. Reason: spacing mistake
Permit me to disagree with the idea that formal writing in 2014 is perfectly appropriate with the conventions in use in 1963.
Or, more elegantly stated, the opinion stated above is one I do not share.
FANBOYS are the coordinating conjunctions: For And Nor But Or Yet So
These can join two independent clauses (and usually are preceded by a comma).
In formal writing, you don't start a sentence with them, as I have with the one that starts with "Or..."
(I don't consider this forum very formal, but it's formal enough that I find texting abbreviations an abomination.)
Last edited by Rover_KE; 17-Aug-2014 at 18:55. Reason: Changing '2014' to '1963'.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.