I would like to take A MINUTE to explain it, however, it might actually take AN HOUR.
I was attacked by A LION; I wish it had been AN ANTEATER.
In this case, if I had written "A HOUR", and a speaker tried to pronounce it, the first vowel sound would run into the second and make it harder to pronounce and understand. Same for "A ANTEATER".
This is English by the way, I have no idea how it applies (or doesn't) to other languages. The A/An decision is based on phonetics as opposed to spelling. So, "... depending on the first character" isn't strictly speaking as true as if it said "... depending on the first phoneme".
I was attacked by A LION; I wish it had been AN ANTEATER.
In this case, if I had written "A HOUR", and a speaker tried to pronounce it, the first vowel sound would run into the second and make it harder to pronounce and understand. Same for "A ANTEATER".
This is English by the way, I have no idea how it applies (or doesn't) to other languages. The A/An decision is based on phonetics as opposed to spelling. So, "... depending on the first character" isn't strictly speaking as true as if it said "... depending on the first phoneme".
For further reference, you can check the below,
https://salesforce.stackexchange.com/questions/21899/starts-with-vowel-sound/21901
I hope you find the above information is helpful. If it does, please mark as Best Answer to help others too.
Thanks.
@Abhishek ,
I understood this concept, that we use 'a' or 'an' while we pronounce the word. But where we can see tha changes if we check that checkbox.
I am not getting the concept yet to use of this checkbox.
But you can check with the salesforce support team for that.