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 London 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 you were from Scotland), but more commonly in London 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 more what are pronounced different to proper english which is from the south