Subject + 在 + Verb
在 (zài) can function as a progressive aspect marker, similar to the English "-ing" form, indicating that an action is currently in progress. It is placed directly before the verb to show that the action is happening at the time of speaking. It is often paired with 正 or 正在 for emphasis.
你别打扰他,他在写作业呢。
Nǐ bié dǎrǎo tā, tā zài xiě zuòyè ne.
Don't disturb him, he's doing his homework.
外面在下雨,带把伞吧。
Wàimiàn zài xià yǔ, dài bǎ sǎn ba.
It's raining outside, bring an umbrella.
她正在跟客户打电话。
Tā zhèngzài gēn kèhù dǎ diànhuà.
She is on the phone with a client.