このナイフの持ち主はいませんか。

this (something or someone close to the speaker (including the speaker), or ideas expressed by the speaker)

knife

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

owner; proprietor; possessor (e.g. of talent, beauty, etc.)

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

to need; to want

indicates a question (sometimes rhetorical); or; whether or not; some- (e.g. something, someone); indicates doubt, uncertainty, etc.; emphatic prefix; suffix forming adjectives or adverbs; in that way