不定冠词a/an的请教教材上说不定冠词a/an,a用于辅音之前,an用于元音之前,英语26个英文字母中,a,e,i,o,

英语26个英文字母中,a, e, i, o, f, h, l, m, n, r, s, x等12个字母的读音是以元音开头的没错,那是音标,不是单个字母,懂么?以下是20个元音音素,开头有元音音素的才是用an的.
8个元音音素及发音规则 元音 /α:/、/כ/、/כ:/、/u/、/u:/、/∧/、/∂/、/∂:/
20个元音音素对应元音音素例子
1) [i:] sea, he, see, piece, ceiling
2) [i]sit, build, miss, myth
3) [e] bed, desk, head,
4) [ ae] bad, land, bank, stamp (a和e中间是连起来的)
5) [a:] car, fast, class, plant, calm, aunt
6) [ ɔ] hot, want
7) [ ɔ:] door, more, sport, ball, warm, author, court, bought, caught
8) [u:] good, who, blue, soup,
9) [u] look, put, women, could
10) [ ʌ] cup, come, blood, rough
11) [ ə:] girl, work, serve, nurse
12) [ə ] cadre, ago, forget, polite, dollar, doctor, famous, Saturday
13) [ei] cake, they, play, eight, great,
14) [ai] bike, die, neither, light, try, find, height, eye
15) [əu ] phone, cold, boat, soul, grow
16) [au] house, town
17) [ɔi ] boy, oil
18) [iə ] dear, idea, deer, here, fierce,
19) [ɛə ] pear, care, there, fair
20) [ uə] tour, poo