Traditionally, Montréal was divided between the French and the English along an east-west border that is the street of St. Laurent. It still is a cultural center of town, with tons of bars, clubs, restaurants, etc. Back in the day, it used to be called the Main. West of the Main was english, east was french. This division was always oversimplified, as the main was (and is) an incredibly diverse mix of cultures. The divide between the French and the English was just the most apparent distinction. To actually break it down here would be confusing, but there are many, many exceptions to that division (such as Outremont, the upper-class french neighborhood, which lies west of St. Laurent). Furthermore, supposedly, that division has broken down significantly as the french have moved west, some hipster anglos are moving east and tons of immigrants are moving in everywhere since La Révolution Tranquile.
When I first came here, I lived about 20 minutes east, by foot, of St. Laurent, in the heart of the Plateau. This was once the working class french district made famous by the plays and books of Michel Tremblay (which I encourage you to read; they are wonderful and translated) who was the first to write in Québécois french. It has now become one of the hottest real estate markets and a trendy place to hang out. But it is still definitely very french. Beautiful little row houses, built around the turn of the century (mine had a stone foundation), divided into multiple dwellings, line the tree-studded streets. Everybody speaks french but more and more of the older generation are moving out, selling their houses to young professionals (yes, the yuppies who complained about our chimney). Still, everybody speaks french, many houses fly Québec flags and it's quite easy to have a conversation with someone on the street about the weather. Even more amazing, little french children play in the streets and back alleys, often unsupervised (I think this is considered child abuse in the rest of North America at this point). They tend to be open and polite, greeting strangers in their high-pitched voices and then getting back to their game.
I have now moved exactly one block west of St. Laurent. We were lucky to find a beautiful old apartment with an incredible rent (and I mean incredible). Even more incredible, when the previous tenants, who'd lived here for 8 years, told the landlord they were leaving and suggested that he could re-write the lease with the new tenants, he didn't want to bother. So now we are west of the Main. And the difference is immediately apparent. All of our neighbours, a mixed bunch, speak fluent english. Only two, so far, are actually french-canadian. The neighborhood is Portuguese. It's actually so portugues that I'm starting to feel like I wished I'd enrolled in Portuguese school as well! You can hear couples arguing in portuguese. We can get insanely good portugues grilled chicken at 3 spots, each within a block of our house. Both hardware stores are owned and run by portuguese.
The immigrants who came before the revolution, came to an english power structure and thus tended to learn english. Their children, who grew up after the revolution tend to be tri-lingual (at least in many cases), though still leaning towards english. When I was over in the french side of the Plateau, probably one in five people would switch to english when I spoke french with them, and they would usually switch back to french after hearing me struggle through. Here, it's very difficult to get them to speak french with me, even the francophones. Part of it is that things are faster and tougher here, more like the way New York used to be and they don't have time to fool around. But I think an even bigger part is that the geographical definitions still hold sway. The culture is deeply rooted in the streets and the Main and parts west are english, so that's what you speak. The people who grew up around here, french, english or portuguese I think see themselves as Montrealers. That's definitely how I see them, cultural mongrels (and I don't mean that perjoratively) who were designed to exist only in this crazy, complex little pocket of a city. It's amazing.
The Main is one of Leonard Cohen's stomping grounds and also contains the theatre where Houdini performed his last show before being punched in the stomach and dying.