了…了【le … le】

Two 了 in one sentence say an action has run for a stretch of time up to now, and usually is still running. The first 了 marks the stretch already done; the second says that is where things stand now.

The verb takes the first 了, then comes the duration, then the object, and the second 了 ends the sentence. English puts the duration last.

• × 我学了中文三年了 → 我学了三年中文了

One 了 or two

Drop the second 了 and the stretch is over.

English draws the same line with "studied" against "have been studying".

A quantity works in place of a duration, with the same sense of "so far". 已经 often goes in front of the verb.

Negation

A stretch of not doing something puts the duration first, then 没 and the verb. The verbal 了 drops, because 没 doesn't allow it; the final 了 stays, because the state still holds.

• × 我没吃了三天饭了 → 我三天没吃饭了