- Computers are important research tools
- Computers are an important research tool.
Does anyone know how to distinguish between (1) and (2)?
There are different ways of expressing the same idea:
- "Computers are important research tools."
This is a straightforward sentence telling us something that computers (all of them) are. Each one is an important research tool, and so all of them "are important research tools."
- "Computers are an important research tool."
This statement, as Ronald Sole points out in his comment, treats computers as a class or set of objects, and equates them, as a unified group, to one thing, "an important research tool."
You could also phrase the concept like this:
- "A computer is an important research tool."
That is a statement about one computer, but since it is framed as a generalization about "a" computer, it isn't limited to a specific one, and must be true about any computer.
Yet another way to put it is:
- "The computer is an important research tool."
Here you are referring to the whole general category of computers collectively as "the computer", and identifying that whole group of entities as "an important research tool."
So to answer your question, yes, some philosophers and other theoretical types would be able to distinguish minutely different shades of meaning between sentences (1) and (2), and also (3) and (4). But for regular people using them for everyday communication, all four sentences really just say the same thing.