when are we supposed to use single quotation marks as opposed to the double ones
' vs "
That depends on the style you choose. In U.S. English double ones are usually used first.* In Commonwealth English it's the single ones which are usually used first. However I don't think things are quite as black & white as that. I'm Australian but I've always used this so-called U.S. style. I'd been using it for so many years before I even knew there was a difference. Then on the other hand, I have seen Australian stuff using Commonwealth style.
* What I mean by "first" this. We alternate between single and double when putting inverted commas within inverted commas.
* What I mean by "first" this. We alternate between single and double when putting inverted commas within inverted commas.
Jim is right. In the US system you use double quotation marks for your main quote, and the single ones to denote a quote-within-a-quote. I believe the British system is the opposite.
It's not really a US vs UK thing. You can use whichever style you like, but consistency is important.
Actually, David, you will almost never see a main quote in single quotation marks in the US.