were is the plural past tense of 'to be' you would use it in any place that you use 'are' when you are in the present tense.
eg. We were late for the meeting.
Where is a preposition, denoting a position.
eg. I can show you where my house is on a map.
eg. Where is my pen?
Now we can put them togethor:
present tense: "I don't know where we are!"
past tense: "I didn't know where we were!"
present tense: "Where are You now?"
past tense: "Where were You last night?
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