To me it sounds like without に, the sentence might be more like "When in Japan, I studied Japanese." and adding に would be somewhat like "When I was in Japan, I studied Japanese." The first one would put more emphasis on studying Japanese (but happen to state where) and the second would be saying what one did in Japan (which was studying). I think I pretty much just reworded what everyone else was saying, in a way, right?
But we say 明日(あした）には, meaning by tommorow."
it helps me to think of に as crosshairs. It zeros in on a specific time. "At" covers this function nicely in English. It's not correct to use it if there is no specific time. 一時に "at 1 o'clock" and 明日 "tommorow" as we don't say "at tommorow". This metaphor can be used for indirect object に as well.
#531 私は、日本にいた時、日本語を勉強した。Good job, Miki.
is there some rules to observed as to use ta-form or impolite form for the preceding clause before the toki under certain conditions? Arigatoo.
ex#531 I feel the Japanese is not natural. It would be 私は日本にいた時日本語を勉強した。or the second user comment 私は日本に行った時に日本語を勉強した。
I see noting wrong with 行った時.Adding に would provide emphasis, such as "I studied Japanese when I went to Japan" (with an implication that happened at no othe time.)
I believe that example one is incorrect. I believe it should be: 私は、日本に行った時に(は)、日本語を勉強した。 You can add or leave out the は depending on how you wish to say it.
Author: your name
Is this usually toki+ni?
You can dispense with either に or は or には, which I think is nore conventional.
tabi i think is often used for repeating things. more like "each time i go to japan" than "when i went to japan".