Determiners are words like the, my, a, his, some, any, those etc. A determiner typically forms the first element in a noun phrase. For example in the noun phrase ‘my father’s house’, the determiner my is the first element in the noun phrase. Similarly in the phrase ‘the little girl’, the determiner the is the first element in the noun phrase.

More examples are given below:

I saw a small child in the store. (Determiner – a)
We bought some sweet mangoes. (Determiner – some)
She has several friends. (Determiner – several)
These mangoes are very sweet. (Determiner – these)

Note that possessive forms such as my, your, our, his, her, their etc., are strictly determiners and not pronouns. Some grammar books, nevertheless, label them as possessive pronouns.

Kinds of determiners

Determiners are of two kinds.
The Group A determiners includes what are called articles, possessives and demonstratives.

Articles: a, an, the
Possessives: my, your, his, her, our, their
Demonstratives: this, these, that, those

Note that we cannot put two Group A determiners together.

Group B determiners

Most of these are quantifiers which indicate something about quantity. Examples are: many, all, both, either, neither, every, much, many, more, most, little, less, least, few, fewer, fewest, one, two, three etc.

Two group B determiners can be used together if the combination makes sense.
Examples are:

All six books
Every ten minutes

Group B determiners can be used directly before nouns, without of.

Have you got any rice? (NOT Have you got any of rice?)

But if we have to put a group B determiner before a noun with an article, possessive or demonstrative, we have to use of

Examples are:

Some books (We do not use of because some is a group B determiner which can be put directly before a noun.)
Some of the books (We use of because a Group B determiner cannot be used directly before a Group A determiner.)

Each child
Each of my children

Most shops
Most of the shops