nearby were two canoes in which they had come to the island为什么句子当中要加in

关系代词在从句中,要做从句的主语 或宾语 或表语 或介词的宾语,就是一定要在从句中做名词可充当的成分
which they had come to the island中which在从句中既不是从句中的主语、宾语,又不是从句的表语,那么一定就要有介词放在which前面.
在which前用什么介词,就可以看"介词+which"在从句中是什么意思酌情选择.
（in ）which they had come to the island中which是指 canoes 的,用in表示采用（某种方式）或（表示位置）在…里面,
这句就可翻译成：他们坐船来到这个岛的.
有时in which=where .