台湾にいたとき私は彼と友達になった。
tbd
While I was in Taiwan, I made friends with him.
I became friends with him while I was in Taiwan.
に
1. at (place, time), in, on, during
2. to (direction, state), toward, into
3. for (purpose)
4. because of (reason), for, with
5. by, from
6. as (i.e. in the role of)
7. per, in, for, a (e.g. "once a month")
8. and, in addition to
9. if, although
tbd
は
1. indicates sentence topic
2. indicates contrast with another option (stated or unstated)
3. adds emphasis
指示句子的主题；表示与另一选项（无论是否明确提及）形成对比；表示强调
と
1. if, when
2. and
3. with
4. used for quoting (thoughts, speech, etc.)
5. promoted pawn
6. indicates a question
表示引用