ies

Ies is one square on the correlative table, the forty-five-word grid that Zamenhof built to replace the scattered question, demonstrative and indefinite words every national language keeps in separate boxes. Read it as a coordinate: the i- row is the indefinite one (some), and the -es column is the possessive one (whose). Some plus whose gives someone's.

Learn the -es column as a column and you get five words instead of one:

• kies - whose (question and relative)
• ties - that one's, the latter's
• ĉies - everyone's
• ies - someone's
• nenies - nobody's

None of the five ever takes -j or -n. They are already possessive, so any plural or accusative ending goes on the noun that follows: mi trovis ies libron, not iesn. That is the one mechanical fact about the column and it holds for all five.

In practice ies is the rarest of them, and it has a slightly literary feel: everyday speech more often says la libro de iu, the book of someone. It earns its place because it is short and because it is unambiguous - ies says the owner is unknown or unimportant, where lia would commit you to a particular person. For how the whole table behaves in running text, see correlatives in prose.