"Tom is French, but he lives in Canada."
Translation:Том - француз, но он живёт в Канаде.
We can replace it by "а" but then we have to remove "он": "Том - француз, а живёт в Канаде. Странно!". It's when the clause means a contradiction.
If we're leaving "он" then this pronoun means that another person lives in Canada, not Tom: "Эндрю говорит, что Том - француз, а он живет в Канаде". Well, maybe not the best example, but it works like that. We can't have the same subjects in the clauses around "a" (unlike "и", "но").
Now here's a weird thing. When i went through this lesson at level 4, I answered Том - француз, но он живёт в Канаде. And that was marked correct, but it offered me Тим - Француз, а живёт в Канаде as an alternative correct answer.
Now, going through it at level 5, I gave Тим - Француз, а живёт в Канаде as my answer - and it was marked wrong.
No, unlike in English using an adjective withoun a noun when talking about one's nationality in Russian makes no sence. You can say "Том - французский инженер" ("Tom is a French engineer") for example, or "этот диван - французский" ("this couch is French"), but not "Том - французский" .
Thank you for the answer. I learned English many years ago, thus for me "Tom is French" sounds strange - I would say "Tom is a Frenchman"; probably therefore I chose the wrong Russian translation. Edit: It seems that grammar has not changed. I found this entry confirming that you really should say "Frenchman": https://books.google.de, it is book isbn 9783125197312 on page 229 - I cannot copy the whole link here