双「了」句（持续） shuāng "le" jù (chíxù)
双「了」句（持续）（shuāng "le" jù (chíxù)）means “V了 + duration + 了: an action has lasted so far and still continues”. With a 了 after the verb AND a 了 at the end of the sentence, plus a duration in the middle, the sentence means the action has gone on for that long UP TO NOW and is expected to keep going. Formation: 主语 + V + 了 + 时量 + (宾语) + 了
V了 + duration + 了: an action has lasted so far and still continues
HSK4Formation
主语 + V + 了 + 时量 + (宾语) + 了
Explanation
With a 了 after the verb AND a 了 at the end of the sentence, plus a duration in the middle, the sentence means the action has gone on for that long UP TO NOW and is expected to keep going. 我学了三年中文了 = 'I have been studying Chinese for three years (and still am)'. Drop the final 了 (我学了三年中文) and it becomes a finished stretch in the past, with no implication of continuing. The final change-了 is what adds 'and it continues'.
Examples
Note
The final 了 = 'still ongoing'. Remove it and the stretch is over.
FAQ
双「了」句（持续） — what does it mean and how is it used?
V了 + duration + 了: an action has lasted so far and still continues. With a 了 after the verb AND a 了 at the end of the sentence, plus a duration in the middle, the sentence means the action has gone on for that long UP TO NOW and is expected to keep going. 我学了三年中文了 = 'I have been studying Chinese for three years (and still am)'. Drop the final 了 (我学了三年中文) and it becomes a finished stretch in the past, with no implication of continuing. The final change-了 is what adds 'and it continues'.
双「了」句（持续） — how do you form it?
主语 + V + 了 + 时量 + (宾语) + 了
双「了」句（持续） — any usage notes?
The final 了 = 'still ongoing'. Remove it and the stretch is over.