在 / 正在 …呢【zài / zhèngzài … ne】

在 before a verb says the action is under way.

正在 and 呢

正在 sharpens it to this exact moment, and 呢 at the end softens the whole thing. All four combinations work, and they differ only in emphasis:

呢 on its own is enough in conversation: 他看书呢.

No tense attached

在 marks an action in progress at whatever time the sentence is talking about, past or future.

在 against 着

在 is the action happening; 着 is the state it leaves. 他在开门 is him opening the door; 门开着 is the door standing open.

Negation

没在 or 不在, with the 正 dropped.

Verbs that refuse it

States do not progress. 是, 有, 知道, 喜欢 and 认识 take no 在 – wǒ我zài在zhī dào知道 is wrong.

Not the same 在 as the place word

Both are zài, and they can appear together: 我在家在做饭 is avoided by dropping one, giving 我在家做饭呢.