Can anyone explain why this is pronounced with a sh sound in the middle?
When "s" is followed by "i", it sounds similar to "ś". (the same with zi (→ź), ni (→ń), ci (→ć)).
Baakamono's correct. You always pronounce the digraphs and trigraphs (si, ni, ci, rz, cz sz, dz, dzi, etc) as a single sound. I've been tripped up by this more than once so it's good to keep in mind!
The sound "sh" you should generally pronunce like in English word: "cash", "brush", "shadow", but a little harder. :) For example: in the word "koszyk" (a basket) you can say: k - like in the English word "can" o - like in the English word "orginal" sz - like in the English word "cash" y - like in the English word "police" k - like in the English word "can". I hope I've helped.
You wouldn't, Ś (= SI) is a very different sound than SZ, so you cannot really write it English way. Although "Poshiwek" will be the closest.
Judging from your level of Russian, I think you should think of Ś/SI as СЬ or Щ and SZ as Ш.
what about SZCZ then? I've always thought (long before I started the course) that that is the Polish Щ but now it seems that Ś is actually Щ
I think that's for historic reasons, that Щ used to be pronounced like that? I don't know, you should be the expert here ;) Anyway I myself have never heard it even remotely sound like "szcz". But theoretically, that's how it should be transcribed into Polish.
And Хрущёв is in fact Chruszczow in Polish, but I've never heard a Russian pronunciation of his surname, so cannot compare.
well I just listened to szcz pronunciation and it sounded like шч (two sounds) and not щ (one sound) at all, so I guess that's another myth "busted"... anyway, it looks like szcz is transliterated as щ in Russian but in some cases it might be also transliterated as шч, for example Шченсны gives 83 000 results in Google, Щенсны 52 200, Шченсный 13 000, and the actually correct Щенсный only 11 900 (that's another issue of whether there should be an й at the end of transliterations of Polish names, the "official" translations have й in the end but it seems to be much more common to not write it (I do that too))
but no one would transliterate Szczecin or Bydgoszcz as Шчечин or Быдгошч
Unfortunately the russian library of phonemes is not as complete as other slavic languages. Ukrainian for example has both the hard and soft ч and ш sounds while Russian does not. (чи in ukrainian is like czy, чі would be like ći I guess? but in russian it would always be the latter case no matter what since a lot of these letters can only make one sound.)
Funnily enough, lots of textbooks for teaching Russian to English speakers claim that щ is pronounced шч 'shch' ... weird how these myths stick around.