分析一句英语的句式结构The light source must therefore be immensely brig
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The light source must therefore be immensely brighter (主系表)

这是主句,the light source为主语,must be 为谓语动词,brighter为表语

therefore 和immensely为状语

if it is to be used as a headlight to illuminate the path

if it is to be used as a signal to others

两个条件状语从句为比较对象.

整个句子的意思:因此这种光源,如果被用作头灯来照亮路,比把它用来作为给别人的信号一定要明亮得多.