If the first letter of the next word is a vowel then you say/write "an" otherwise it's "a"
FTFY.
A vs. AN
Always use 'a' except if the next word
1. starts with a vowel or
2. starts with an 'h' that is not pronounced (IOW, it's as if the word is starting with a vowel)
This means that 'a' horse/historian/hotel is what we say in the US, but some English would use 'an' (h)orse/(h)istorian/(h)otel.
The English say 'a' herb where Americans say 'an' (h)erb.
Let me set this straight, as it is not vowels and silent Hs that are the determining factors:
A vs. ANIf the next word begins with a vowel
sound then the preceding word is an. In all other cases it's a.
Examples:- Please pass me a banana.
- I would love have an apple to eat.
- Yesterday, I closed on a house.
- Call me in an hour.
- The word map is spelled with an "M", not with an "N" or with a "C".
To wrap it up, it all depends on the beginning
sound of the following word, regardless of its spelling. (No need to keep track on rules which types of Hs and vowels etc.)