I thought I knew about commas before I started looking into them. It was a learning curve.
Looking into. . .
But, and other little words that join things together, and. . .
The rule for but is the same as that for and, or, nor, for, so, and yet. (These are called coordinating conjunctions, but you don’t need to remember that.)
We’re back to essential versus nonessential information here. If the conjunction follows a main clause that makes sense on its own, it’s added information, so add a comma:
David sprinkled more salt, but he still couldn’t eat the spinach.
If both clauses are necessary to make any sense, no comma is needed:
David sprinkled more salt but still couldn’t eat the spinach.
There is another way of looking at and and but that I find easier to remember.
If clauses have a subject on both sides of the conjunction, there should be a comma.
I phoned Emma earlier, but I haven’t heard back from her.
I phoned Emma earlier but haven’t heard back from her.
If the second clause assumes the same subject without naming it... or him or her… (in this case, I ) you don’t add a comma.
Peter got up and made his bed before heading down for breakfast.
Peter got up, and he made his bed before heading down for breakfast.
(A case of adding a comma when you add a subject? Or am I pushing this tip too far?)
What rule applies here?
I didn’t watch the film, because it starred Alfredo Bloggins.
I didn’t watch the film because it starred Alfredo Bloggins.
You might be forgiven for thinking that this is one situation where the discredited ‘pause’ myth might apply. However, there is a grammatical reason.
In the first sentence – with the comma – because refers to the whole of the first clause. The point is that I didn’t watch the film; the reason I didn’t watch is added information. The first part of the sentence will make sense without giving a reason why.
The whole purpose of the second sentence is to explain why I didn’t watch the film. This because is essential to the meaning of the sentence – it’s the whole point of it. Since this information isn’t added, it wants no added comma.
Another helpful resource. . .
The most exhaustive comma crib in one place must be Purdue OWL; another brilliant resource all-round, at . . .