"Does anyone know why English people developed non-rhotic accent? How come they didn't pronounce 'r' even though the word obviously has an 'r' in the spelling?"
Nobody really knows, although it seems to be a fairly common phonological shift among European languages, existing is dialects of not only English, but also to lesser or greater degrees in some forms of German, Portugese, Spanish and French.
"1. When the hell did American accent become different from British accent?"
Basically American and British English stopped developing together around 1750, when the English stopped emigrating in any largeable quantities (the Irish and the Scottish are quite a different story). Both GenAm and RP are most likely related to the same mid-Eighteenth century dialect of the language. In fact, if you took a GenAm accent and changed nothing except making it non-rhotic, you'd probably notice that it isn't as radically different as RP as you might think.
These are a really good trio of links that give a very detailed account for how British and American English diverged:
http://www.phon.ucl.ac.uk/home/wells/x202-7as6-lecture.pdf
http://www.phon.ucl.ac.uk/home/wells/x202-8-9-as-7-lecture.pdf
http://www.phon.ucl.ac.uk/home/wells/x202-8-lecture-new.pdf
"2. Which area's accent is the origin of American accent? Can you trace it?"
It really depends a lot on what you mean by American accent, since there are obviously different varieties. But I'll assume you mean General American.
GenAm is a form of English spoken in the Midwest in the first half of the twentieth century. Interestingly, although GenAm is widespread through much of the West and East these days, many of the dialects of the actual midwest are diverging greatly from this model, either influenced by the Northern Cities Vowel Shift or, in the southern part of the region, by Southern English.
As far as there being a specific place of origin, I guess the case could be made for Western New England being the birthplace of 'standard' American English, since it is generally postulated as being the birthplace of Northern Midwestern English.
"3. Why do Canadians speak more like Americans despite the fact that they are still a member of the British Commonwealth?
Shouldn't they speak British accent?"
Commonwealth status obviously has nothing to do with linguistics.
Nobody really knows, although it seems to be a fairly common phonological shift among European languages, existing is dialects of not only English, but also to lesser or greater degrees in some forms of German, Portugese, Spanish and French.
"1. When the hell did American accent become different from British accent?"
Basically American and British English stopped developing together around 1750, when the English stopped emigrating in any largeable quantities (the Irish and the Scottish are quite a different story). Both GenAm and RP are most likely related to the same mid-Eighteenth century dialect of the language. In fact, if you took a GenAm accent and changed nothing except making it non-rhotic, you'd probably notice that it isn't as radically different as RP as you might think.
These are a really good trio of links that give a very detailed account for how British and American English diverged:
http://www.phon.ucl.ac.uk/home/wells/x202-7as6-lecture.pdf
http://www.phon.ucl.ac.uk/home/wells/x202-8-9-as-7-lecture.pdf
http://www.phon.ucl.ac.uk/home/wells/x202-8-lecture-new.pdf
"2. Which area's accent is the origin of American accent? Can you trace it?"
It really depends a lot on what you mean by American accent, since there are obviously different varieties. But I'll assume you mean General American.
GenAm is a form of English spoken in the Midwest in the first half of the twentieth century. Interestingly, although GenAm is widespread through much of the West and East these days, many of the dialects of the actual midwest are diverging greatly from this model, either influenced by the Northern Cities Vowel Shift or, in the southern part of the region, by Southern English.
As far as there being a specific place of origin, I guess the case could be made for Western New England being the birthplace of 'standard' American English, since it is generally postulated as being the birthplace of Northern Midwestern English.
"3. Why do Canadians speak more like Americans despite the fact that they are still a member of the British Commonwealth?
Shouldn't they speak British accent?"
Commonwealth status obviously has nothing to do with linguistics.