Final answer:
To create a negative adverbial sentence from the given sentence, one would use inversion to start with the negative adverb 'never' followed by the auxiliary verb and subject, as in 'Never had I met him in such an informal situation.'
Step-by-step explanation:
To turn the sentence "I almost never met him in such an informal situation" into a negative adverbial sentence, one could rephrase it to emphasize the rarity of the event by altering the position of the negative adverb 'never'. An example of a negative adverbial sentence could be: "Never had I met him in such an informal situation." This construction uses inversion, placing the negative adverb 'never' at the beginning of the sentence for emphasis, followed by the auxiliary verb 'had' and the subject 'I', which adds a sense of surprise or unexpectedness to the fact that the meeting in an informal situation almost did not occur.