Translation:It always snows during the winter in Canada.
It would be 'tio' because it's a noun. 'tiu' is the 'that' of 'that one'. It goes with 'kiu' (which). One would not ask “kiu estas..” unless there is a set selection of things to choose from, just like one would not ask “which one is...” in English without a set selection to choose from. Likewise one would only use 'tiu' if there is a set selection, as it can be thought of as an answer to 'kiu'. Personally I think it would be easier to remember if we didn't use 'that' for both in English (perhaps we should use 'thich' to answer 'which'... Or maybe not)
I'm guessing, but ĉiam-dum plus a verb seems to take the action out of the verb's tense and place it into a sort of abstract state. So ĉiam neĝas dum vintro as in this example becomes "it always snows during winter." Mi ĉiam kantos dum festoj. = "I will always sing during festivals" Sofia ĉiam ludis dum lecionoj = "Sophia always played during lessons" and so on. It seems that the implication is that this always happens, but is not necessarily the only thing that happens. There are sunny days during Canadian winters, I might only sing one or two songs at each festival, and Sophia might have played sometimes during each of her lessons, but not others.
I hope this is correct.
Ah, the stereotypes! And we Canadians all live in igloos and all use dogsleds too, di'ncha know. Everyone wears parkas or plaid lumberjack shirts and owns a pair of snowshoes too . . . NOT.
Snow depends entirely on where you are in Canada -- the Pacific northwest is mostly snow-free, and with climate change proceeding apace, the "snowline" moves further north every year. And the Arctic is melting fastest of all, so that polar bears are beginning to starve for lack of access to ice floes for hunting.
Where I am, on Vankuvera Insulo, it would be more accurate to say, "Cxiam pluvas en vintro." Cxi tie, cxiam pluvas en oktobro, novembro, decembro, januaro, februaro, marto, aprilo kaj junio. En majo, la vetero estas suficxe bone, aux bela, sed la pluvo venas denove in junio.
Wow, who would have thought that a random test in about snow in Canada would have me find another person living en Vankuvera Insulo (and my first learning of how to spell Vancouver)! Finally I know I'm not alone. And FYI - it is random things like this, funny or not, that when I was growing, we had a foreign exchange student from Mexico that actually believed it always snowed in Canada and we lived in Igloos before she got here. But yes, I agree, "ĉiam pluvas dum vintro en Britia Kolumbia".
Occasionally there is an invite to join a group on duolingo. You might benefit from accepting one of those invites.
And I remember the foreign exchange student from Brazil while I was living in Fairbanks, Alaska. He was in awe to discover a green state when he arrived in late August. And unprepared when the snows came in October.