Here's how:
Uncountable: When referring to the general concept of a population (like the population of a country), it's uncountable. You can't say "one population" or "two populations" in this sense.
* Example: The population of India is very large.
Countable: When referring to specific groups of people within a larger population, it becomes countable.
* Example: There are many populations of endangered animals.
* Example: The research focused on three different populations of students.
In short:
* Uncountable: "Population" is uncountable when it refers to the general number of people in a place.
* Countable: "Population" is countable when it refers to distinct groups of people within a larger population.