"Tennis" is the object of the verb "play" in the phrase "play tennis".

在“打网球”这个短语中，“网球”是动词“打”的宾语。

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This word is a transitive verb. We can use a noun or noun phrase as its direct object.

这是一个及物动词，我们可以用名词或名词性短语直接作其宾语。

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The Case Condition principle requires that a noun phrase have Case and Case is assigned by V(verb) or P(preposition) to the object position, or by AUX(auxiliary) to the subject position.

格条件要求名词词组必须有格，而格被动词或介词指定在宾语位置，或者被助动词指定在主语位置。

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