A determiner is a small word that sits in front of a noun and specifies it — it tells the reader which one, how many, whose, or how definite the noun is. Determiners don't describe a noun the way adjectives do; they point at it.
Without the determiner, each noun would float in the air: book on the table feels incomplete, brother leaves the reader asking whose. That pointing job is exactly what determiners do.
Determiner vs adjective
Both sit in front of a noun, but they work differently.
An adjective describes a quality: red, tired, expensive. A determiner specifies identity or amount: the, my, two. If both appear, the determiner always comes first.
Another difference: you can usually remove an adjective and the sentence still works. Remove the determiner and it often falls apart.
The main types
Most determiners fall into six groups. Each group has its own lesson later — here they are only mapped.
Articles
The smallest and most frequent determiners: a, an, the. They mark a noun as specific or nonspecific.
Demonstratives
This, that, these, those — they place a noun near or far from the speaker, in space or in time.
Possessives
My, your, his, her, its, our, their — they show who the noun belongs to.
Quantifiers
Words that answer how much or how many: some, any, many, much, few, little, several, a lot of, no.
Distributives
Each, every, all, both, either, neither — they describe how a group is divided or taken together.
Numbers
Numbers are determiners too, whether they tell you how many (one, two, three) or in what order (first, second, third).
One determiner per noun
The normal rule is simple: one determiner per noun phrase. You can't stack them freely.
A handful of fixed combinations break this rule, and they're worth memorising as set phrases rather than as a pattern to reproduce.
Beyond these, if you want to add a second idea, use a different structure — a book of mine instead of the my book.
A determiner is often required
English is strict about this: a singular countable noun almost always needs a determiner in front of it. You cannot drop it the way you can in many other languages.
Plural and uncountable nouns are more flexible. They can stand alone when the meaning is general.
But as soon as you talk about specific dogs or specific water, a determiner comes back: the dogs next door, this water, my water bottle.
Common mistakes
Missing determiner before a singular countable noun
Wrong order with an adjective
Two determiners stacked together
Determiner and noun disagree in number