There are three articles in English: a(n), the, and ‘zero article’ Ø. Articles belong to the grammatical class deictic (‘determiners’ in traditional grammar). The deictic is part of the nominal group and takes up the same slot as ‘this’ ‘these’ ‘those’ ‘my’ ‘your’ etc.
Nominal group structure
|six||cute||koalas||sitting in the tree|
You can see that you can choose from a range of deictics, including ‘the’. This is why we can’t say ‘the my koala’ because you choose just one deictic: ‘the’ or ‘my’.
Deictic means ‘pointer’ which means that the deictic positions the noun so that you can see it from a certain point of view. ‘Those’ tells us that the koalas are far from the speaker. ‘These’ tells us that the koalas are close to the speaker. But ‘the’ is much more difficult (We will see how ‘the’ points in part 3). ‘My’ tells us that the koala belongs to the speaker.
Of course the choice of deictic depends on the type of noun. You can say ‘my koala’ and ‘my koalas’ and you can say ‘this koala’ but you can’t say ‘these koala’. This is because the noun ‘koala’ is singular. So you see that the use of articles depends on whether the noun is plural or singular.
Here is the general rule of thumb:
A singular count noun cannot be on its own; you need to give it an article or a plural marker.
Koala eats grass. ✘
A koala eats grass. ✔
The koala eats grass. ✔
Koalas eat grass. ✔
Of course each correct example has a different meaning. We’ll talk more about this in part 2.