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We’re currently in Japan. I’m talking with my mom about a Japan tour we joined.

And I say:

Remember we went on a tour of Japan before I moved here.

Or

Remember we joined a tour of japan before I moved here.

Are both okay?

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#1 is OK.

#2 is not.

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Remember we joined a tour of japan before I moved here.

joined a tour is not a natural thing to say.And it sounds like the tour was already underway.

You could say

eg We took a tour / signed up for a tour

Capital J

Students: Are you brave enough to let our tutors analyse your pronunciation?
Comments  

Why not?

You can say:

We joined Mr. and Mrs. Johnson on their tour of Japan.

Students: We have free audio pronunciation exercises.
 Clive's reply was promoted to an answer.