I'm not sure If I've understood perfectly your question ...but I think that depends on the word you write after the article "a". For example:
Does an elephant have a trunk?
Does a student need to wear uniform at school?
Altough the examples are not really good, what I meant to explain is that if the noun that follows the "a" starts with a vowel, it should wear the -n, if not it would be the simple "a"