N-COUNT Adverbs such as "fortunately" and "perhaps" which apply to the whole clause, rather than to part of it, are sometimes called sentence adverbs. 句子副词
Once it's clear that "however" is just an introductory adverb, the sentence makes sense.
“However,” 在这里只是一个引导性的副词，这个句子就是有意义了。
The "modifier" (adverb or adjective in a sentence) is not required, usually.
通常不需要“修饰语”（句子中的副词和形容词）。
The header of a DP is an adverb, and the whole phrase plays the adverbial role in a sentence.
副词短语以副词为核心词，同时整个短语在句子中，仍然为副词的功能。
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