"Ĉiam neĝas dum vintro en Kanado."
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.