The sentence structure of "David Atchison may have been the president of the United States for one day" is a declarative sentence in the present tense. The subject of the sentence is "David Atchison," and the verb is "may have been." The predicate of the sentence is "the president of the United States for one day." The sentence contains a noun phrase "the president of the United States" which acts as a complement of the verb phrase "may have been." The prepositional phrase "for one day" functions as an adverbial modifier of time, indicating the duration of Atchison's hypothetical presidency.
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