了⋯⋯了
le …… le
Expressing ongoing duration with double "le"
A second 了 at the end says the action is still going on.
Verb + 了 + Duration + 了
How it works
他在北京住了两年了 — he's lived in Beijing for two years and still lives there. Drop the final 了 and he's moved away.
That single character is the whole difference between English's "lived" and "has lived".
The final 了 is the change-of-state one, bringing the situation up to the present.
Examples
tā zài běi jīng zhù le liǎng nián le
他在北京住了两年了。
He's been living in Beijing for two years.
wǒ xué le liǎng nián zhōng wén le
我学了两年中文了。
I've been studying Chinese for two years now.
wǒ děng le bàn gè xiǎo shí le
我等了半个小时了。
I've been waiting half an hour.