The indefinite pronoun 'someone' is singular; it takes the place of a noun for one unknown or unnamed person.
plural-singular nouns
The medical term "dysphagia" (trouble swallowing) is a noun, but it is not singular or plural.
Criterion is the singular. The plural is criteria.
'Hypotheses' is plural; the singular would be 'hypothesis'.
The word "someone" is singular. It refers to an unknown or unspecified person.
It is singular hence the word 'one'.
It is singular hence the word 'one'.
Mitochondria is already plural. The singular form is Mitochondrion.Mitochondria IS plural. The single form is mitochondrion.
Organisms is the plural of organism
phalanges
Sonría / sonrían // sonríe / sonreíd. (Formal singular/plural//informal singular/plural)
The noun simplicity is the singular form; the plural form is simplicities.