Use a when the indefinite article comes before a word beginning with a consonant sound:
- a toy
- a book
- a house

Use an when the indefinite article comes before a word beginning with a vowel sound:
- an operation
- an idea
- an apple

Exceptions

It's important to note that not all nouns that begin with a consonant letter begin with a consonant sound. Always trust the sound, not the letter, when applying the a vs. an rule. Remember, it's about pronunciation. Consider the examples below:
- a university
- an honor