nothing We shall be having rain,rain,and nothing but rain.

【We主语】【 shall be having谓语】【 rain,rain,宾语】 and nothing but 【rain.】
and nothing but rain=and （we shall be having)nothing but rain.承前省略
nothing but ：除了…以外什么也没有；只有,不过,不外
Verse in which there is nothing but the beat of the metre furnished by the accents of the polysyllabic words we call doggerel.Verse is not that.
诗句中没有其他的东西,只有节奏,这节奏是由多音节的词造成的,我们叫打油诗,诗歌不是这个.
As you can see,it's nothing but a pattern of four bars plus four bars plus four bars plus four bars.大家可以看到,它只不过是一种模式,若干段四个小节的叠加