Generic: whole class
Forms: the + singular (without reference); Zero + plural/mass (default); a + singular (not just one)
teachers are old
Indefinite: unidentified subset
Forms: a + singular (default); Zero + plural/mass (refer a subset)
a cat
Definite: identified subset
Forms: the + singular (with reference); the + plural/mass; this/my + ...
the population of Monsoon, this cat
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Fours main kinds of Referers:
Articles: a(n), the, Zero
Demonstratives: this, that, these, those
Possessives: my, her, your, ...
Inclusives: all, either, both, ...