Q From Libby Hall: Why do we pronounce the words sugar and sure as though they began with the letters sh?
A The early history of sugar is obscure, especially the way it was said. But variations in the way it was spelled in old documents suggest strongly that the u was said as a diphthong, /juː/ , very much the way we now say the word you, and that the initial s was said like that, not as sh. Sometime in the Middle English period the initial letters su shifted to the pronunciation they now have.
If you relax the mouth and tongue somewhat when you are saying the older form, your pronunciation shifts to the modern one, as you’ll realise if you try out the two sounds in turn; the modern version is actually rather easier for slack-jawed English speakers to say.
The same change happened with other words, such as sure, and also to words in which the sound occurred in the middle, such as pressure and nation. By the time this shift in pronunciation was taking place, the spelling of the words had already become fixed, so the way they’re now written conflicts with the way they’re said. But that’s only too common in modern English!