Usages of lǎoshī
你 是 老师 吗？nǐ shì lǎoshī ma?
Are you a teacher?
我 不 是 老师。wǒ bú shì lǎoshī.
I am not a teacher.
Note how bù (falling tone) changes it's pronunciation to bú (rising tone) when followed by a falling tone.
这 个 人 不 是 老师。zhè gè rén bú shì lǎoshī.
This person is not a teacher.
这 个 老师 在 家。zhè gè lǎoshī zài jiā.
This teacher is at home.
那 个 老师 今天 太 忙 了。nà gè lǎoshī jīntiān tài máng le.
That teacher is too busy today.
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