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

i was in japan . we can not use after been "in"

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

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.

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.

-u use the first one in past simple when u wanna talk about a specific event in a specific time that happened in the past, so the first one must include a specific time line such as i was in japan 2 yrs ago

-u use the second one in present perfect to talk about your life experince like ''i've been to japan once'' but present perfect doesn't need to include a specific time line