mereka

In Indonesian, the word mereka never changes form. Whether it acts as a subject, object, or possessive, it remains exactly the same (there is no case inflection):

• Subject: 'They' (e.g., Mereka makan - They eat).
• Object: 'Them' (e.g., Saya melihat mereka - I see them).
• Possessive: 'Their' (e.g., Rumah mereka - Their house).

Number: Pronominal mereka is strictly plural. Do not copy English singular 'they' for one person whose gender is unknown; use singular dia or ia, or beliau when respect is intended.

Unlike aku or kamu, mereka does not have its own unique suffix. The suffix -nya can refer back to a plural group in suitable contexts; TBBI itself gives plural-topic uses such as Para petani sawahnya diserang hama wereng. The full word mereka makes a plural possessor explicit. Do not impose a blanket formal ban on every plural-related -nya.

Mereka in the Passive Voice
With a third-person agent, the di- passive with oleh is a standard choice (e.g., Buku itu dibaca oleh mereka). The object-voice form Buku itu mereka baca is also established in published formal Indonesian, especially when the patient is the topic or appears in a relative clause. The two forms package information differently; the bare verb in object voice is not merely colloquial.

A Different Word: mereka as a Verb
A separate homograph mereka, formed from the root reka, is an active verb meaning to compose, devise, plan, imagine, or guess. It is less frequent than the pronoun and also occurs as mereka-reka. Thus Mereka sedang mereka undang-undang baru can mean 'They are drafting a new law.' Do not imitate the awkward collocation mereka alasan; say mereka-reka alasan or mengarang alasan for 'make up an excuse.' Context and capitalization distinguish the initial pronoun from the following verb.

Those who...
A very common pattern for English speakers to learn is mereka yang.... While English uses the demonstrative pronoun 'those' (as in 'those who...'), Indonesian uses the personal pronoun mereka combined with yang. For added emphasis, it can be reduplicated as mereka-mereka (e.g., Mereka-mereka yang... - Those specific people who...).

Usage Note: The ordinary reference is human, but TBBI also allows animals or personified things in fictional narration. For inanimate objects in formal writing, it is more common to use unmarked plurality or itu as a subject. When referring to inanimate objects as an object ('them'), the suffix -nya is the standard choice. Additionally, in very formal situations, it is more respectful to use beliau-beliau or titles like Bapak-Bapak (Gentlemen) or Ibu-Ibu (Ladies) instead of mereka when referring to a group of respected people.