bởi

Because the use of bởi as a passive agent marker is a borrowed syntactic structure, it carries specific nuances regarding register and usage:

1. Disambiguation: 'Because' vs. 'By'
In everyday Vietnamese, bởi is most frequently encountered as a shortened form of bởi vì (because). When bởi means 'by', it almost always directly follows a verb (usually in a sentence already containing được or bị). When it means 'because', it usually sits at the beginning of a clause, followed by a noun or pronoun subject.

2. Register and Frequency
Bởi (meaning 'by') is highly frequent in modern written Vietnamese, particularly in academic writing, journalism, literature, and commercial advertising. It is heavily associated with 'translationese' (translated prose). However, it sounds unnatural and overly 'Westernized' in casual, everyday spoken Vietnamese.

3. Contrasting with Native Passives
In everyday speech, native speakers strongly prefer the zero-preposition passive: Favourable Passive With An Agent or Adverse Passive With An Agent. For example, instead of saying được viết bởi anh ấy (written by him), a native speaker will naturally say được anh ấy viết.

4. The 'Heavy Noun Phrase' Rule (End-Weight)
One of the primary functional reasons writers choose bởi over native passive structures is to handle long, complex agent phrases. In a native passive, the agent sits in the middle of the sentence. If the agent is a 'heavy' noun phrase (e.g., 'a panel of international experts'), placing it in the middle disrupts the flow. Using bởi allows the writer to shift this long agent phrase to the end of the sentence, improving readability.

5. Contrasting with 'do'
Both bởi and Formal Passive With do can translate to 'by' for passive agents. However, their syntax for introducing agents is opposite. As an agent marker, do is placed before the verb, whereas bởi is placed after the verb. (Note that do can appear after a verb when it means 'due to', but not when marking a passive agent).

6. The Prescriptive Rule on Human Agents
Some prescriptivist grammar books and formal education standards dictate that bởi should not be used with human agents. While modern media shows this restriction is frequently ignored (with sentences like được viết bởi tác giả being ubiquitous), it remains a strict rule in formal testing. For formal writing or exams, Formal Passive With do does not have this restriction and is the safest alternative for introducing human agents.

7. The Requirement for 'được' or 'bị'
When using bởi as an agent marker, the passive markers được or bị are required before the verb. Directly translating English reduced relative clauses by dropping them (e.g., writing một cuốn sách viết bởi Nguyễn Nhật Ánh instead of một cuốn sách được viết bởi Nguyễn Nhật Ánh) results in unnatural translationese and should be avoided.