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"I was in Japan." vs. "I've been in Japan." - which sentence is grammatically correct?

Thank you.
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Both sentences are grammatically OK.

"I was in Japan last year." One specific event in the past. Usually an adverb of time is in the sentence.
"I've been in (to is also OK) Japan." One or more times in the past. The idea that you have been there earlier is more important than the specific event.
Comments  
i was in japan . we can not use after been "in"
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Can you say: "I've been to Japan."?
Can you also say: "I've been in love."? Or, should you never say "in" after "been"?

Thank you.
i was in japan
 AlpheccaStars's reply was promoted to an answer.
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Thank you AlpheccaStars!
So, should I say: "I've been in Japan twice." (if I've been there more than once)? And: "I was in Japan 5 years ago." (If I don't want to mention that I've been there more than once)? Also, please tell me if you see grammatical errors in this post, even if they got nothing to do with my question.

Thank you.
You could say "I've been TO Japan twice."

That sounds more natural than "I've been IN Japan twice." The "to" emphasizes the nature of taking a trip, versus being in the country itself.