Taiwan is part of China .It is ---- the southeast of our cou

on the southeast of+地点 指相邻并且接壤
to the southeast of+地点 指相邻不接壤,如中国和日本
in the southeast of+地点 指在……领域之内的东南部
from the southeast of+地点 指从……领域之内的东南部
at the southest 用的不多 应该是指具体的地点 比如说 A river lies in the southest of our town.
台湾和中国要用in,虽然隔海但是台湾属于中国领土,因此不可用to.