“正在” “正” “在” as adverbs, are all used to indicate the proceeding of an action or the continuation of a state. “正” focuses more on time, “在” emphasizes the status, while “正在” indicates both time and status.
1. “正” is used to express that “something has not taken place yet, but it’ll happen soon”. The verb like “想” or “要” often follows “正”.
你打电话的时候，我正要起床。(Nǐ dǎ diànhuà de shíhòu, wǒ zhèng yào qǐchuáng.) I was about to get up when you called.
>>I am still in bed, but I will get up very soon.
事情发生的时候，他正想回家。(Shìqíng fāshēng de shíhòu, tā zhèng xiǎng huí jiā.) When it happened, he was about to go home.
>>He has not gone back home yet, but he would go very soon.
2. “正” can be followed by a locality phrase, such as “在家” “在学校” “在医院” and etc. “正在” and “在” CANNOT be used in this way.
我正在医院看医生。(Wǒ zhèngzài yīyuàn kàn yīshēng.) I am seeing a doctor in the hospital.
他们正在办公室开会。(Tāmen zhèngzài bàngōngshì kāihuì.) They are having a meeting in the office.
3. “正” can be followed by “从”, while “正在” or “在”
他正从家去公司开会。(Tā zhèng cóng jiā qù gōngsī kāihuì.) He is going from home to the company for a meeting.
4. “正” usually does not connect with a single Verb (especially monosyllable Verbs), while “在” and “正在” have no such limitation.
We are discussing.
✘ 我们正讨论。(Wǒmen zhèng tǎolùn.)
√ 我们在讨论。(Wǒmen zài tǎolùn.)
√ 我们正在讨论。(Wǒmen zhèngzài tǎolùn.)
√ 我们正讨论着呢。(Wǒmen zhèng tǎolùn zhene.)
In the form “Subject + 正 + Verb/Adjective + 着 + 呢”, if the verb or adjective is a monosyllable, “着” has to be used; if it’s disyllable, “着” is optional.
I’m having dinner.
我正在吃。(Wǒ zhènɡzài chī.)
我在吃。(Wǒ zài chī.)
我正吃着呢。(Wǒ zhènɡ chī zhe ne.)
我们正讨论（着）呢。(Wǒmen zhèng tǎolùn (zhe) ne.) We are discussing.
When there is a preposition phrase before or after disyllable verb or phrase, or there is a preposition phrase after a monosyllable verb, “正” could be used without “呢” in the end.
我去的时候，他正从楼上下来。(Wǒ qù de shíhòu, tā zhèng cóng lóu shàng xiàlái.) He was coming downstairs when I went.
我正走向超市，你在哪？(Wǒ zhèng zǒu xiàng chāoshì, nǐ zài nǎ?) I am walking towards the supermarket, where are you?
“正 + verb/adjective + 着” is often used as the former clause of a compound sentence, indicating that one action happens while another action is in progress.
大家正忙着，客人已经到了。(Dàjiā zhèng mángzhe, kèrén yǐjīng dàole.) The guests have arrived while everyone is busy.
你打电话的时候，孩子们正哭着。(Nǐ dǎ diànhuà de shíhòu, háizimen zhèng kūzhe.) The children were crying when you called.
5. “在” is used to emphasize that an action or state continues. The word like “一直”, “继续”, “总”, “又” or “还” is often used with it, while “正” and “正在” CANNOT.
我在继续学习汉语。(Wǒ zài jìxù xuéxí hànyǔ.) I am continuing to study Chinese.
这些天他一直在找工作。(Zhèxiē tiān tā yīzhí zài zhǎo gōngzuò.) He has been looking for work these days.
“在……（呢）” “正在……（呢）” “正……呢” can be used interchangeably to indicate that an action is in progression and continues.
I was doing laundry while he was eating.
他吃饭的时候，我在洗衣服（呢）。(Tā chīfàn de shíhòu, wǒ zài xǐ yīfu ne.)
他吃饭的时候，我正在洗衣服（呢）。(Tā chīfàn de shíhòu, wǒ zhèngzài xǐ yīfu ne.)
他吃饭的时候，我正洗衣服呢。(Tā chīfàn de shíhòu, wǒ zhèng xǐ yīfu ne.)
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