The rule of thumb is to use "an" before a word that starts with a vowel *sound*, not just a vowel letter.
Here's why:
* Pronunciation is key: We use "an" for smoother pronunciation. Saying "a apple" sounds awkward, while "an apple" flows more naturally.
* Vowel sounds, not just letters: Sometimes words that start with a consonant letter actually have a vowel sound. For example:
* "hour" starts with the "h" but sounds like "ow" (a vowel sound).
* "honest" starts with "h" but sounds like "on" (a vowel sound).
Examples:
* "an" is used before:
* an apple (starts with vowel sound)
* an hour (starts with vowel sound)
* an honest person (starts with vowel sound)
* "a" is used before:
* a unicorn (starts with consonant sound)
* a European (starts with consonant sound)
* a university (starts with consonant sound, even though the "u" is a vowel)
Important Note:
There are some exceptions to this rule, especially with words that start with "h" that are pronounced with a strong "h" sound, like "history" or "hotel". In these cases, "a" is often used.
Let me know if you'd like more examples or have any other grammar questions!