Plural superlatives
 

I've been trying to find some information regarding plural superlatives or whatever this is called. By this I mean the grammar used in the following sentence: "The three most important things are..." Normally, we're taught the the superlative is for one thing, but of course we also use sentences like "the three most important things" and "the second most important thing", etc. I'm trying to find out information about this use of the superlative to teach my students. Anyone could refer me to any place in books or the web where I can find more information about this? Thanks.

Re: Plural superlatives
 

Who says we use the superlative to refer to one thing? It's a rule that makes no sense (and one I don't think I've ever seen??). If you forget it, then there's nothing to "find out". What is it you want to know exactly? The superlative expresses the idea that something is "at the top" of a category. So you can just as easily talk about the top three things in a category (the three biggest/most important etc) as you can talk about just the one at the top (the biggest/ most important). In fact you can think of it as :

The most important thing to keep in mind is... = the one most important thing - but we don't bother to include "one" because it's redundant. The use of the singular tells us we're talking about one thing only.

If on the other hand we say : The most important things to keep in mind are ... then we know there's more than one, but it's vague - we don't know exactly how many we're talking about. So if we want to specify we add the number : The three most important things are ...

But there's nothing special about this. The superlative is just an adjective form and as such acts like any other adjective. Compare :

It's certainly a big problem : It's the biggest problem we have
Those are important points : Those are the most important points.
He made three important points : The three most important points he made were ..

I really don't think there's anything to explain here - but it may be that I've not fully understood your question. If so, apologies. Can you be more precise?