The progressive aspect and the continuous aspect
The Chinese language does not employ tenses to indicate the time (present, past or future) of and event’s happening; instead there are aspects, which indicate the stages of event. For example, en event can be viewed as being in progress, in continuation or having been completed.
A. The progressive aspect
The first part of this chapter focuses on the use of the progressive aspect, which indicates an action in progress.   Since  there are no tenses in Chinese, an action in progress might be happening in the past, at present or in the future. The actual time of the event’s happening is indicated by a time phrase, an independent clause or is not mentioned at all if it is understood in the context.

1. Basic form of the progressive aspect
There are three basic ways to show an action in progress:
正 在 Verb 呢 Example
√    √ optional 下午我正睡觉( 呢 ),他就打电话来了。
Xià wǔ wǒ zhèng shuìjiào (ne), tā jiù dǎ diànhuà lái le.
In the afternoon I was sleeping and he called.
√ √ optional 爸爸在睡觉( 呢 ),别去吵他。
Bàba zài shuìjiào (ne), bié qù chǎo tā.
Your father is sleeping. Don’t go and disturb him.
√ √ √ optional 大地震发生的时候,我正在吃晚饭( 呢 )。
Dà dìzhèn fāshēng deshíhou, wǒ zhèng zài chī wǎnfàn (ne).
When the big earthquake occurred, I was eating dinner.

(a) 正+verb+( 呢 )
A   sentence  using ‘正+verb+( 呢 )’ normally doesn’t stay alone; therefore, its usage is quite limited. It is most frequently used to express the time when another action takes/took place. It is also used with an action that does not last (see (f) below for more on this).
今天下午我正看电视 呢 ,他就来了。
Jīntiān xiàwǔ wǒ yhèng kàn diànshì, tā jiù lái le.
This afternoon he arrived when I was watching TV.

昨天我到他家的时候,他正从家里出来( 呢 )。(出来 is an action that does not last)
Zuótiān wǒ dào tā jiā de shíhou, tā zhèng cóng jiā lǐ chūlai ne.
Yesterday when I arrived at his home, he was coming out of his house.

(b) When  呢  is used
When the optional  呢  is used, the speech sounds more casual
(Situation: Zhang and Wang are chatting over the phone. It’s a very informal situation.)
张:你在做什么?
Zhāng: Nǐ zài zuò shénme?
王:我(正)在看电视( 呢 )。
Wáng: Wǒ (zhèng) zài kàn diànshì (ne).

(c) When  呢  is not used (1)
Since   呢   is   a   modal   particle   that   appears   at   the   end   of   a   sentence , it should not be used mid-sentence.
李:你们在做什么?
Lǐ: Nǐ zài zuò shénme?
王:我在吃水果,他在写功课( 呢 )。
Wáng: Wǒ zài chī shuǐguǒ, tā zài xiě gōngkè (ne).
Li: What are you (plural) doing?
Wang: I am eating fruit; he is doing homework.
(Improper: 我在吃水果 呢 ,他在写功课 呢 。)

(d) When  呢  is not used (2)
Likewise,  呢  is never used when the action in progress is in a relative clause.
(正)在跟王老师说话的那个人是谁?
(Zhèng) zài gēn Wáng lǎoshī shuōhuà de nà ge rén shì shéi?
Who is the person that talking with the Teacher Wang?
(Improper: 在跟王老师说话 呢 的那个人是谁?)

(e) 正在/在+verb+( 呢 )
Although ‘正在+verb+( 呢 )’ and ‘在+verb+( 呢 )’ are generally viewed as being interchangeable, ‘正在+verb+( 呢 )’ carries a more emphatic tone than ‘在+verb+( 呢 )’. Therefore, when the speaker mentions a specific moment of the time, 正在 tends to be used.
昨天下午他来我家的时候,我正在洗澡;给他开门的人是我妹妹。
Zuótiān xiàwǔ tā lái wǒ jiā de shíhou, wǒ zhèng zài xǐzǎo; gěi tǎ kāimén de rén shì wǒ mèimei.
When he came to my house yesterday afternoon, I was taking a bath; The person who opened the door was my younger sister.

张:你在想什么?(你正在想什么? would be less common  since  no specific time is mentioned.)
Zhāng: Nǐ zài xiǎng shénme?
王:在想王老师要我们回答的那个问题。
Wáng: Zài xiǎng Wáng lǎoshī yào wǒmen huídá de nà ge wèntí.
Zhang: What are you thinking about?
Wang: (I am) thinking about the question Teacher Wang wanted us to answer.

昨天老师叫到我的时候,我正在做白日梦( 呢 )。
Zuótiān lǎoshī jiào dào wǒ de shíhou, wǒ zhèng zài zuò báirì mèng (ne).
Yesterday when the teacher called on me, I was daydreaming.

(f) Actions that do not last
Verbs in the ‘正在+verb+( 呢 )’ and ‘在+verb+( 呢 )’ structure must indicate actions that can last. For actions that do not last, use ‘正+verb+( 呢 )’ to emphasize that the action is in progress.
老师正进教室的时候,火警的警铃响起来了。(进 is considered an action that does not last.)
Lǎoshī zhèng jìn jiàoshì de shíhou, huǒjǐng de jǐnglíng xiǎng qǐlái le.
When the teacher was entering the classroom, the fire alarm went off.

老师正在骂学生的时候,下课的铃声响了。(骂 is an action that can last.)
Lǎoshī zhèng zài mà xuésheng de shíhou, xiàkè de ling shēng xiǎng le.
When the teacher was scolding her students, the bell of  the   end  of class rang.