1. It refers to a singular, unspecified noun.
* Example: I saw a dog in the park. (We don't know which dog specifically)
* Example: She bought a new car. (We don't know the specific car)
2. It comes before a consonant sound.
* Example: I ate a sandwich. (The word "sandwich" starts with a consonant sound)
* Example: He wrote a poem. (The word "poem" starts with a consonant sound)
3. It's used in front of words that start with a silent "h" and are pronounced with a consonant sound.
* Example: He read a history book. (The "h" in "history" is silent, and the word starts with a consonant sound)
4. It's used in front of abbreviations that are pronounced with a consonant sound.
* Example: She works for a BBC. (The abbreviation "BBC" is pronounced with a "b" sound)
5. It's used in front of numbers that begin with a consonant sound.
* Example: He bought a 100-year-old house. (The number "100" starts with a consonant sound)
Important Note: You use "an" instead of "a" when the noun following it starts with a vowel sound.
* Example: I saw an eagle. (The word "eagle" starts with a vowel sound)
* Example: She read an interesting book. (The word "interesting" starts with a vowel sound)