Pronunciation of articles in English

[Igel]

3. We reduce vowels in about the same way.

With some differences by context.

But I must say, [ ən ] sounds utterly unnatural to me and /ə/ is disallowed in most standard English at least under stress (perhaps only primary stress). Maybe /_[n] matters and what you cite is a British variant (I remember by Czech students of English having a hellish time with /kænt/ which they insisted on pronouncing as [kɐnt] which was a rather bad error, as you can imagine. I imposed /kænt/ on them when they had trouble with [kɑ:nt]. Even [kɐ:nt] just came off as emphatic--even to Estuary and RCP speakers.

Also note how /n/ only occurs because of historically mis-parsing and generalization, not through n-insertion when /a/ was in hiatus with another /a/: "The apron" but "an apron" (*apron), or "the orange" v "an orange" (*narange or thereabouts, cf. Sp. naranja) and probably some Latin or Greek borrowings, me thinks. There's nothing like that for < the >.

Now /æn/ is generalized to /_# æ, where # is a morpheme boundary and [ən] ~ [æn] are in complementary distribution when unstressed, at least in every dialect of American (and British) English that I'm familiar with. But what do I know?