[gonzotactics]

We cannot say , I live in the Toronto.
but we can say, I live in the Toronto area.
Why?
Gonzo

[gonzotactics]

I think I got it. Proper nouns become proper adjectives and thus are able to be used with def articles.

[susan53]

Yes, more or less. In English, "the" is not used to qualify a name - here Toronto, but also eg Susan and Microsoft (both of which could be preceded by a definite article in some other languages - for example Italian).

In your second example, Toronto is only acting as an adjective - the article is qualifying area. not Toronto. Compare eg
A : There's a lot of industry in the area.
B : I sorry, I don't follow. Which area?
A : The Toronto area

The only exceptions to this are names which are made up of adjective + noun - eg the United States, the Netherlands (nether = low), and so can be used with the article without breaking any rules, and a couple of odd ones like The Gambia - though even here there's a theory that it derives from the word "Gambura" meaning "place of the king" - which would explain the article.