I was sad for my country back then when I could see no Afghan athletes in the opening ceremony.
In this sentence, what is part of speech of 'when'? Is it a relative modifying 'back then' which is a noun, or is it a conjunction leading a subordinate clause?
It is a subordinator, a conjunction.
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I was sad for my country back then [ when I could see no Afghan athletes in the opening ceremony ].
Traditional grammar treats this "when" as a subordinating conjunction. But forget that for a moment.
In modern grammar, the analysis of the bracketed expression is in fact quite problematic. One solution is to treat it as a PP (preposition phrase), w