是 vs 当 (shì vs dāng): to be vs to serve as
是 (shì) states identity or classification, equating two nouns (e.g., 我是老师 'I am a teacher' — a fixed status). 当 (dāng) indicates taking on a role or serving as something, often implying a temporary or functional position (e.g., 我当老师 'I work/serve as a teacher'). Use 是 for permanent attributes; use 当 for active duties or temporary assignments.
The core distinction is that 是 (shì) is a copula used to equate subjects with nouns or adjectives, expressing inherent identity or classification. 当 (dāng) means 'to serve as' or 'to act as', focusing on the performance of a role or duty, often temporarily. For professions, 是 states the current identity (a static fact), while 当 emphasizes the active work or the assumption of the role. Both can be used for jobs, but the nuance differs: 是 is neutral, while 当 carries a sense of function or temporary assignment.
When to use each
Use 是 to state identity, classification, or inherent attributes. It equates the subject with a complement, indicating a permanent or essential quality. For example, to say 'I am a student' (我是学生) or 'This is a book' (这是一本书). 是 is also used for dates, times, and emphatic structures like '是...的'.
是 is stative and does not imply active role-taking. When used with professions, it simply states the occupation as a fact, not necessarily the current activity.
Use 当 to indicate taking on a role, serving as something, or acting in a capacity, usually with a temporal or functional quality. Common with jobs, duties, or positions one assumes (e.g., 当老师 'work as a teacher', 当班长 'serve as class monitor'). 当 can also mean 'to become' in the context of choosing a path (e.g., 我想当医生 'I want to become a doctor').
当 implies an active engagement in the role; it can also mean 'to be in charge of' or 'to treat as' in certain contexts (e.g., 当他是朋友 'treat him as a friend'). As a preposition, 当 means 'when' (e.g., 当我在家的时候 'when I am at home'), but that is a separate usage.
At a glance
| 是 | 当 | |
|---|---|---|
| Core meaning | Identity/classification (copula) | Role/function (serve as) |
| Permanence | Typically permanent or essential | Often temporary or functional |
| Used with professions | States the job as identity (我是医生) | States the job as active role (我当医生) |
| Negation | 不是 (bù shì) — not be | 不当 (bù dāng) — not serve as |
| Can follow 想 ('want')? | No (想是 is ungrammatical for 'want to be') | Yes (想当 'want to become/serve as') |
| Time expression placement | Typically before 是 (我今天是老师) | Before 当 (我今天当老师); cannot be after 当 |
Examples
- 是我是老师。Wǒ shì lǎo shī.I am a teacher.States permanent identity or occupation.
- 当我今天当老师。Wǒ jīn tiān dāng lǎo shī.I am the teacher today.Temporary role; time expression precedes 当.
- 当他想当医生。Tā xiǎng dāng yī shēng.He wants to become a doctor.Use 当 after 想, not 是.
- 是她是我的同班同学。Tā shì wǒ de tóng bān tóng xué.She is my classmate.Identity/classification.
- 当他当班长两年了。Tā dāng bān zhǎng liǎng nián le.He has served as class monitor for two years.Functional role over time.
- 当✗我当老师今天。→✓我今天当老师。✗ Wǒ dāng lǎo shī jīn tiān. →✓ wǒ jīn tiān dāng lǎo shī.I am the teacher today.Time expression cannot follow 当; place before verb or at clause start.
Common mistakes
- Using 当 instead of 是 for permanent identity: '我当中国人' is wrong; should be '我是中国人' (I am Chinese).
- Using 是 instead of 当 after 想 to express 'want to become': '我想是老师' is ungrammatical; use '我想当老师'.
- Placing time expressions after 当: '我当老师今天' violates word order; use '我今天当老师' or '今天我当老师'.
- Using 是 for a temporary role when 当 is more natural: '我是今天的班长' is acceptable but '我当今天的班长' better emphasizes the temporary function.
- Confusing 当 (role) with 当 (when): '当我老师' would mean 'treat me as a teacher' or 'when I teacher'? Context matters; avoid ambiguity by using 作为 for 'as' when needed.
FAQ
- When do I use 是 vs 当 for professions?
- Use 是 to state someone's profession as an identity (e.g., 他是医生 'he is a doctor'). Use 当 to emphasize the active work or temporary assignment (e.g., 他当医生二十年了 'he has worked as a doctor for 20 years'). Both can be correct, but the nuance differs.
- Can 是 be used for temporary roles?
- Yes, but it simply states the current state (e.g., 我是今天的班长 'I am today's class monitor'). 当 is more common for temporary roles because it highlights the functional aspect (e.g., 我当今天的班长).
- What about 当 as a preposition meaning 'when'?
- 当 can also be a preposition meaning 'when' or 'at the time of' (e.g., 当我在家的时候 'when I am at home'). This usage is distinct from the verb 'serve as' and does not compete with 是.
- Is there a difference in negation between 是 and 当?
- Yes. 是 negates as 不是 (bù shì) — 'am/are/is not'. 当 negates as 不当 (bù dāng) — 'do not serve as' or 'will not be'. For example, 我不是老师 'I am not a teacher' vs 我不当老师 'I am not working as a teacher' (refusing the role).