Plural Rule: us Becomes i
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
A singular ending in u s forms its plural by changing the u s to i.
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Bronchus becomes bronchi; nucleus becomes nuclei; alveolus becomes alveoli.
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Virus and sinus are exceptions. Virus takes viruses; sinus takes either sinuses or the unchanged sinus.