zhè gè rén bú shì lǎoshī.

In Mandarin, demonstratives 这/那 normally must be followed by a measure word (classifier) before a noun: 这 + 个 + 人. 个 is the default measure word for many nouns, including 人.

• 这个人 = this person (standard).
• 这人 exists in casual speech and can sound colloquial or a bit blunt, like “this guy.” For learners, use 这个人 as the safe, standard form.

Chinese is Subject–Predicate.

• Subject: 这个人
• Predicate: 不是老师 (verb/copula 是 with negation 不 before it, and the noun 老师).
  Negation 不 is placed directly before verbs/adjectives, so you get 不 + 是 → 不是.

Tone sandhi: 不 (bù) changes to rising tone bú when it comes before a fourth-tone syllable. 是 (shì) is fourth tone, so 不是 is pronounced bú shì.

• Keep bù when the next syllable is not fourth tone: e.g., bù gāo.
• In short, emphatic answers, speakers may keep bù even before a fourth tone (e.g., answering with bù shì for emphasis).

Use 不 for general, habitual, or present/future negation and with the verb 是. The negative of 是 is always 不是.
没 is used for past/completed events and to negate 有 (to have):

• 他不是老师 = He is not a teacher.
• 他没来 = He didn’t come.
• 他没有老师 = He doesn’t have a teacher.

No. 是 is needed when linking a subject to a noun (copula with nouns).

• Correct: 他是老师。/ 他不是老师。
  With adjectives, you typically don’t use 是: 他很高 (He is tall).

Two common ways:

• Add 吗: 这个人是老师吗？ (Is this person a teacher?)
• Use the A-not-A pattern: 这个人是不是老师？
  A negative expectation question would be: 这个人不是老师吗？ (Isn’t this person a teacher?)

Use 这些 (these) and keep the noun without a plural ending:

• 这些人不是老师。
  Avoid 老师们 here; 们 is mainly for pronouns or when addressing a known group (e.g., 老师们好 = Hello, teachers).

这个人 can sound a bit blunt. For politeness, use the respectful measure word 位 and a title:

• 这位先生/女士 (this gentleman/lady)
• 这位老师 (this teacher)
  If you don’t know the person’s role, 这位 on its own is also common in context.

Chinese leaves definiteness to context. 他不是老师 can mean either. To force “the teacher,” specify:

• 他不是那位老师。
• 他不是我们班的老师。
  These make it definite.

Sentence-final 的 can nominalize or add emphasis in elliptical contexts:

• 他不是老师的。 often appears when contradicting or contrasting prior info (“He’s not a teacher, actually”).
  In a neutral standalone statement, 他不是老师 is more straightforward.

Add the change-of-state particle 了 after the predicate:

• 这个人不是老师了。
  This signals a new situation compared to before.