Zài

Zài (在) is a common Chinese character and function word used primarily as a preposition meaning at, in, or on, and as an auxiliary indicating ongoing action. In standard Mandarin it is pronounced zài with a fourth tone.

As a locative and temporal preposition, 在 introduces a location or a point in time before a noun phrase or before a verb. Examples include: 我在家。书在桌子上。我们在三点开会。These uses cover where something is or when something happens, such as 在学校, 在路上, 在下午三点, or 在明天.

As a continuative or progressive marker, 在 can precede a verb to express an ongoing action, similar to the English present progressive. Example: 我在吃饭。你在做什么？ In many contexts, this sense can be strengthened with 正在 or 正在做 to emphasize ongoing activity: 我正在吃饭。

In addition to its core functions, 在 forms part of many common compound words and expressions that relate to presence, existence, or relevance to a topic. Examples include 在场 (present at an event), 在座 (present and seated), 在线 (online), 在意 (care about), and 在位 (in office). These compounds illustrate how 在 integrates location, existence, and topical relevance.

Grammatical note: In modern Mandarin, 在 is one of the most frequent functional words and interacts with nouns, verbs, and aspect markers to shape meaning about where something is, when it occurs, or whether an action is in progress. Its usage is widespread across speech and writing, making it essential for basic sentence construction and expression of time and place.