Tell me please,why some people say wonderful (like wUnderful),Sunday(like Soonday), Russia(rUsha)?

It depends on what part of the country you're from. If you're from nearer to 런던 in the South you'd say wonderful wun-derful and Russia rush-a, but if you're from up north, you'd pronounce Sunday soon-day (if 당신 were from Scotland), but 더 많이 commonly in 런던 it id said sun-day. It really all depends on your accent.

Hope that made sense :)

It's just the different accents that people have around the country :)

i'm from the north, yorkshire infact and in yorkshire we say
sundy for sunday
wunderful for wonderful
rusha for russia
and many many 더 많이 what are pronounced different to proper english which is from the south