1、【Twenty green bottles hanging on the wall.这是一个动名词短语作定语,修饰前面的名词】
And 【if one green bottle should accidentally fall,条件状语从句】 there【 would be 谓语】【nineteen green bottles主语】【 hanging on the wall.定语】.
2、And if【 one green bottle从句主语】【 should accidentally fall谓语】.
句子意思:
墙上挂着二十个绿色瓶子.
如果一个绿色瓶子偶然掉下来,只有十九个绿色瓶子挂碍墙上.
祝你开心如意!