As long as you are talking about one population, it is thought of as one or the same as uncountable, and yes, that is funny- because we try to count populations all the time. Here is how you use is with a single population: What we are talking about IS the whole population. What we are talking about is only a portion of the population. IS goes with uncountable things- a single population is one of those. But: You can have different populations. You can found how many populations you have. If you have more than one you can say things such as: Both of these populations are.... All these populations are... Three of these populations are.. Most of these are populations that... Therefore you can count populations when there are more than one.

population = countable noun It can be uncountable under special circumstances, but basically it is countable. I'm afraid your teacher was wrong, as are indeed most ESL teacher.

popuation is uncountable, usually: 1. population as the subject, the Predicate Verb should be single form. The world\'s population is increasing faster and faster 2.when subject is percentage, the Predicate Verb should be plural form. About seventy percent of the population in China are farmers Sometimes, it can be used as plural form, also can put a indefinite article " a" before it, to express a total population, here, the population as a whole. New York is a big city with a population of over 10 million. Many parts of the world, which once had large populations and produced plenty of crops, have become deserts.