I live in a very old town which is surrounded by beautiful woods
中的关系代词 which 指代主句中的先行词 town.把它改写为下面的两个简单句就可以看清楚:
I live in a very old town.
The town is surrounded by beaubiful woods.
第二句话的主语 the town 就是第一句的 an old town,变成定语从句时,这个名词正好用关系代词 which 来指代.
翻译:我住在一座古老的城镇里,城镇的四周都是美丽的树林.
(按原来顺序翻译即为:我住在一座非常古老的被美丽的树林环绕的的城镇里.)