the great fire in london,in which a large part of the city w

这位网友,这个长句可以“还原”成两个简单句,分别是：
1、 the great fire in London broke out in 1666；
2、A large part of the city was destroyed in the fire.伦敦城有很大一部分在这场火灾中被毁.注意：在这场火灾中,英语的介词用的是in.
再把这两个简单句合成一个复杂句,就是：
The great fire in London,in which a large part of the city was destroyed,broke out in 1666.
这就是介词in 的由来.
希望对你解开疑问有所帮助.