形容词作表语,而不是状语.分词结构可做状语,所以用法不一样.“george was exhausted after a day's work”里面的“exhausted”是形容词,而“exhausted by a day's work”里面的“exhausted”是动词的过去分词,所以后面用“by”.
如果用“after”,那么应该这样说：“Being exhausted after a day's work,he took some tablets to help him feel better.”那么在这个句子中,“exhausted”同样是形容词.