Example sentences
wǒ我zhèng正zài在chī吃fàn饭。
I'm eating.
tā他zhèng正zài在kàn看shū书。
He's reading.
About this word
正 means just, exactly, right at, the 正 of 正好 and 正是, and 在 means to be located at. Stack them and you get right at this moment, the marker for an action in progress. Either half can also work alone, so 我在吃饭 and 我正吃饭 both mean I am eating, with 正在 as the emphatic version. The sentence often ends in 呢 for colour (我正在看书呢). The construction never takes 了, because the action is still unfolding, and it refuses state verbs, which is why 我知道 has no 正在 form.
The story behind the character
正
正 is 止, a foot, above a line. Some oracle bone forms show that foot aimed at a city wall, so the idea was marching straight at a target, which gave both to move on something and to set it right. Xu Shen wrote 正, 是也, straight, correct. 正在, right now, keeps the sense of being exactly on the spot.
在
才 on top, 土 (earth) below. A plant just breaking through the soil. That's existence. Xu Shen wrote 在, 存也: "to exist." So "我在家" is literally "I exist, here at home."