They could hear David screaming as they sat in the waiting room, powerless to do anything except wait.
句中的 powerless... wait 应如何分析?
单纯的形容词或形容词短语除了充当定语、主语补足语(表语)和宾语补足语之外,还可以充当状语。
A. 原因状语
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Too nervous to reply, he stared at the floor. 他紧张得不能答话,只是凝视着地板。
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Glad to accept, the boy nodded his agreement. 这个男孩很高兴接受,点头表示同意。