如果是限制性定语从句,在翻译的时候就放在先行词前面翻译,译成“.的”
例如：These are the trees which were planted last year.这些是去年种的树.
The cakes which my mom cooked were tasty.我妈妈做的蛋糕很美味
如果是非限制性定语从句,就可以翻译成“这...”
例如：He drove carelessly,which was very dangerous.他开车很大意,这是很危险的.
当然多数情况下你可以灵活一点翻译.