One alveolus is a singular, tiny air sac of the lung, surrounded by a network of capillaries, and through which oxygen and carbon dioxide are exchanged. A cluster of such air sacs are alveoli (plural of alveolus).
The singular form of the word flagella is flagellum.
Criterion is the singular form of criteria.
The word 'duo' is the singular form; the plural form is duos.
The singular form of the word "señores" is "señor."
That is the singular form; the plural form is those.
'Jackhammer' is the singular form.
The singular form is commando.
The word dog is the singular form.
The singular possessive form of jury is jury's
The word "biopsies" is the plural form of the word. The singular form is "biopsy."
The singular form of Sioux is "Sioux." It is both the singular and plural form of the word.
The singular form of the word "possum" is possum. The plural is possums.