What is the difference between niwa and dewa? にはとではのちがいはなんですか
So i understand that ni is for existence. E.g私はねこがいる。 (I have a cat)And that de is for something happening in a location or by.でんしゃでがっこうにいく。(i go to school by train)うちでまんがをよむ。( at home i read manga)However when combining niwa and dewa im slightly confused. For example, igrisu dewa eigo wo hanasu. ( in england (they) speak english) however couldnt i equally say igrisu de eigo wo hanasu. However the second one is incorrect to say. Can someone please explain.Please also add some more examples.

Question

Answer

very roughly degfining:

+++「に」：target place of action
   (where you do to, where you are)
    「ここにいる（I'm here)」
　「ここに来た（I came here)」
    「あなたにこれをあげます(I'll give you this)」
　「私に何の用？*(what's your business with me?)

+++「で（＝に＋て）」：place of action (where you do)
    「ここで遊ぶ（I play here)」
    「家で読む（I read at home)」
    「電車で行く（go by train)」

+++「は」: particle indicating 'theme/issue' of the sentence/paragraph.("talking about")
   For detail, please see:https://www.italki.com/question/424283

**** The diffrence between "Igirisu dewa" and "Igirisu de"

"Igirisu dewa eigo wo hanasu". :
  -> *(Talking about 'in England', it's to speak english)
  ->  in england (they) speak english.

"igirisu de eigo wo hanasu. "

No issue is clarified.
  Then this means: 
   "Speaking English in England"

e.g.
"igrisu de eigo wo hanasu nowa tanoshii"
  Speaking English in England is fun.

"igrisu de eigo wo hanasu? (watashi niwa) Zettai muri!"
  Speaking English in England? No way (for me)