"Ш" твердая по произношению, а "Щ" мягкая по произношению

ш - sh / щ -shch

HI. What difference do you interested in? Sound, using? "Щ" - sounds like any english sh (shine, shoe etc) "Ш" - it sounds hardly and more like to any russian sounds.

Both these sounds in Russian is called "щелевые шипящие" (fricative hushing sounds). Pronouncing ""ш" you blow out air through the gap between a palate and AN END of tongue, but for "щ" sound you should make a longer (or deeper) gap between tongue and palate (along the length of at least a half of tongue). The longer gap makes softer sound - Щ. P.S. "В чём разница?"

"Sure" starts with щ sound. It is very soft and loud. "Shine, shape, shake" start with ш sound. It isn't soft and it's like whisper.