y' is followed by the noun indicating who ACTIVELY is doing the surrounding..
就是说如果surround后面的名词是主动的做包围这个动作,用by
比如 we are surrounded by the mountain.
如果surround后面的名词被动的做包围这个动作,用with.或者说这个主语和surround前的主语是形成一个整体,就用with
the cake is surrounded with berries