I learned おつかれさまでした (otsukaresama deshita) as a form of good bye, used in the office. But I often hear my Japanese colleagues say おつかれさまです (otsukaresama desu) instead, or at least that's what I think I heard. Is the た just silent, somehow? Or if both are valid, what's the difference between the two forms?
Both are valid expressions and the difference is very subtle.
お疲れ様です can be used for both "Hello" and "Good bye" (to tell someone leaving from workplace), and お疲れさまでした can be used for "Good bye" (to tell someone leaving from workplace).
Notes: -It is very frequent that you start Japanese business email to colleague with "お疲れ様です". Presumably working in Japan is always tiring... -When you use it for meaning "Good bye", there are no perceptive difference in them for native speakers. Grammatically the person has done something tiring, so "お疲れさまでした" might be more correct, but we don't care indeed.. (Well the difference might be more controversial and you need to consult linguist for rigorous understanding)
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