The quick answer

Use a/an with a singular countable noun, some in positive sentences with plural or uncountable nouns, and any in questions and negative sentences. Say Do you have any apples?, not Do you have some apples?

The rule

Countable nouns can be counted and have a plural form: one apple, two apples. Uncountable nouns have no plural form and cannot be counted directly: water, rice, money, advice, information — never waters or informations.

Use a/an only with a singular countable noun: an apple. Use some in positive sentences with a plural countable noun or an uncountable noun: some apples, some water. Use any instead of some in questions and negative sentences: Do you have any apples? I don't have any water.

Form

Examples

• I need an apple.
• She wants some water.
• Do you have any eggs?
• We don't have any milk.
• They need some advice.

Common mistakes

• ❌ I need a water. → ✅ I need some water. Water is uncountable, so it cannot take a/an.
• ❌ Do you have some milk? → ✅ Do you have any milk? Questions normally use any, not some.
• ❌ She gave me some advices. → ✅ She gave me some advice. Advice is uncountable and never takes -s.
• ❌ I don't have some time. → ✅ I don't have any time. Negative sentences normally use any, not some.

Practise these patterns often, and soon choosing between a, an, some and any will feel completely natural in everyday conversation.