Common problems with Count ◦ Uncount Nouns

Substances as count or uncount nouns
Substances are usually uncount nouns:

Would you like some cheese?
Coffee keeps me awake at night.

but they can also be used as count nouns:

I'd like a coffee, please.
I'd like a cup of coffee.

They sell a lot of coffees.
They sell a lot of different kinds of coffee.

They had over five cheeses.
They had over five types of cheese.

This is an excellent soft cheese.
This kind of soft cheese is excellent.

Some nouns have both a count and an uncount form. Their meanings are closely related:

George had hopes of promotion.
We should always have hope.

There's a danger of avalanches on the mountain.
Some people enjoy danger.

Some nouns have two meanings, one count and the other uncount:

Can I have a glass of water?
I cut myself on some glass.

Is English a difficult language?
Linguistics is the study of language.

The Times is an excellent paper.
It's made of paper.

Other nouns like this are:

Some uncount nouns end in –s.
They look like plural count nouns, but they are not.

Nouns like this generally refer to:

Subjects of study:
mathematics, physics, economics, etc.

Activities:
gymnastics, athletics, etc.

Games:
cards, darts, billiards, etc.

Diseases:
mumps, measles, rabies, etc.

Economics is a very difficult subject.
Billiards is easier than pool or snooker.