Not sure how old this discussion is, but this is an important distinction to make. Cathain is the question when, nuair is the conjunction when.
Consider the different meanings of when in these examples: -When do you eat? I eat when I run. -Cathain a itheann tú? Ithim nuair a rithim.
(If you consider that nuair once came from a contraction of "an uair" [the hour/the time], you can explain this as "Ithim an uair a rithim" - I eat /the time that/ I run i.e I eat /when/ I run)