An is an interrogative particle.
It causes eclipsis, when followed by an ecliptable consonent.
In all tenses apart from the past:
In the past tense:
Its spelling and general pronunciation are the same as those of the definite article, an.
The initial vowel sound is /ə/ (sometimes nearer /ɛ/ or /ɪ/ when conditioned by following palatelisation) rather than /ɒ/.
This difference, which might seem trivial on first consideration, is similar to that between the two vowel sounds in the English word 'upon'. Try swapping those and see how it sounds.