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Statements

In positive sentences, we use:

A / AN + singular countable noun SOME + plural countable noun
There is a cat in the garden.

There are some birds in the trees.

I’ll have a cup of coffee.

He has some great ideas.

Note: Though 'any' is mainly used in negative sentences and questions,
we can use it in positive statements with the meaning 'not important
which'.
- You can have any three items of clothing you like for $30.
- They should be here any minute.

Negatives

In negative sentences, we use:

A / AN + singular countable noun ANY + plural countable noun
There isn’t a dog in the garden.

There aren’t any birds in the tree.

He hasn’t got a car.

We didn’t take any pictures.

See more examples:

To make pancakes…

… you need a frying pan.
… you don’t need an electric mixer.
… you need some plates.  
… you don’t need any chopsticks.

Questions

In questions, we use a/an, any and how many:

A / AN + singular
countable noun

ANY +
countable noun

HOW MANY + plural
countable noun

Is there an apple on the
tree?

Are there any chairs in the
garden?

How many books are there?