Thank you for your time.
This is a very common mistake for people, even natives, writing in English. I did a quick Google search, and it seems there are several sites offering practice questions and general help with the issue.
I suggest you try googling 'were where', 'were where problems', 'were where exercises' and so on. I'm sure you'll find what you are looking for.
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To me, "where" and "were" are comletely different animals and shouldn't be mixed together.
where = question word regarding location; the vowel is pronounced as it let; even though it is spelled wh..., the initial sound is actually that of hw.
were = the past tense of the plural forms of to be. The vowel is pronounced as schwa (a relaxed the)
Where were you last night should have two distinct pronunciations.
She also finds reading out loud to be helpful, especially if she is doing so when she is conscious of the fact that she is checking herself specifically for the usage of these two words. She does sometimes read "were" as "where", so vigilance is required!