I happened to be upstairs at dusk when the window was open.这个句子是定语从句还是状语从句.
1 定从.先行词 at dusk 为时间,when 从句修饰作用.译为：我恰巧在那个窗子开着的黄昏上楼了.
2 状从.when 从句修饰主句.译：我碰巧在楼上,当时窗子是开着的.