Countable nouns can be singular or plural.
Which implies singular nouns is a type of countable nouns. Other parts in that tutorial justifies this.
However, Collins says
Singular nouns that are uncountable are used without a determiner when you are making a general reference.
Could singular nouns be uncountable? If yes, in what kind of situations they could be?
It seems that it's meaningful to distinguish singular or plural in terms of countable nouns. Which means that if a word is uncountable, it's not countable, therefor it's neither singular nor plural?
Is my understanding correct?