Yod dropping - Why is there a distinction in the pronunciations of "sewn" and "hewn"?

The yod, /j/—the sound at the beginning of the word yes—has dropped out of very many words beginning with alveolar and post alveolar consonants in English. This has happened more widely in American English than in Southern Standard British English (SSBE), which still retains yods in many words beginning with /t, d, n/.

Just behind your upper teeth - you can feel it with your tongue - there is a little shelf-like part of your mouth. It slopes slightly upwards. Behind that your mouth suddenly arches upwards to form the roof of your mouth. That shelf-like part you can feel there behind your teeth is called your alveolar ridge. The alveolar consonants are the ones that we make with our tongue touching this shelf. The alveolar consonant in English are /s, z, t, d, n, l/. Postalveolar consonants are ones we make with the tip of the tongue right at the edge of the alveoar ridge, or just behind it. The postalveolar consonants in English are /r, ʃ, ʒ, tʃ, dʒ/ (only one word in English starts with /ʒ/, the word genre).

The following words all used to have yods in in American and British English, but don't for many modern speakers of different Englishes:

• suit /su:t/
• Zeus /zu:z/
• tune /tu:n/ (SSBE still has a yod here for most speakers; /tju:n/)
• dune /du:n/ (SSBE still has a yod here for most speakers; /dju:n/)
• news /nu:z/ (SSBE still has a yod here for most speakers; /nju:z/)
• lute /lu:t/
• rude /ru:d/
• chute /ʃu:t/
• chew /tʃu:/
• juice /dʒu:s/

The word hewn does not begin with an alveolar or postalveolar consonant, and so we would not expect the yod to have disappeared in the modern word. However sewn like the word suit begins with the alveolar sibilant /s/ and so we might reasonably expect the yod to have disappeared here.

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^{Readers may find this Wikipedia article interesting. Many of the word here were lifted from that article.}