Although our language is rich in culture, I dislike it some days for the arbitrariness I sometimes perceive. Here is one such example:

• An unusual service
• A usual service

The general rule is that ‘an’ is used preceding a word beginning with a vowel and ‘a’ precedes a word beginning with a consonant. I know, I know, this exception is caused by the ungodly sound of “an usual service.” I just find it mildly irritating. I can only imagine an English learner trying to figure this out.