The difference between a, an, some, and any can be quite confusing for many people.
Most simply “a” and “an” are used with singular countable nouns while “some” and “any” are used with countable and uncountable plural nouns.

A vs. An
“A” is used with singular nouns that begin with a consonant.[1]https://www.ecenglish.com/learnenglish/lessons/when-use-a-some-any-elementary
For example:
- Can I have a
cup of water?
- There is a
dog in the pool.
- This is a
cool car.
“An” is used with singular nouns that begin with a vowel.
For example:
- Can I have an
enormous cup of water?
- There is an
excited dog in the pool.
- This is an
ancient car.
Some vs. Any
“Some” is used with plural countable and uncountable nouns in positive statements.
For example:
- There is some
cheese in the refrigerator.
- I have to do
some homework.
- She has some
work to do.
“Any” is used with plural countable and uncountable nouns in negative statements and questions.
For example:
- I don’t have
any cheese in my fridge.
- She doesn’t
have any homework.
- Is there any
work to do?
Exceptions with Some and Any
Normally with questions, you use “any” for plural countable and uncountable nouns. However, there are 3 major exceptions to this rule.[2]https://www.grammar.cl/Notes/Some_Any_A_An.htm
- Offer
Would you like some coffee?
- Ask for
Can I have some coffee?
- Suggestion
How about we go out for some coffee?
Practice Exercises
Complete the following sentences with either “a”, “an”, “some”, or “any”.
How did you do?
If some of these questions were tricky don’t worry — this is one of the topics that confuses English learners the most.
Sometimes reading alone isn’t enough. A private one on one lesson can clear up any confusion quickly and help you use this grammar naturally and confidently.

References
