Why can't 了 be used to indicate a completed action that has yet to come? 了 is a particle that indicates the perfective--the verb has no interior composition. This is different from the perfect aspect in English which focuses on the result of an action in the past that may or may not have interior composition. So why is it that I cannot say "明年，我学了中文" to indicate that by next near I would have learned Chinese? In Russian, the perfective aspect can be used in a future tense.
i think it is because of that we use 了 to express that the status is really exist,mostly it is the status of something has done or over/ended/finished.as you can not make sure a thing has not happened definitely happen in the future,so you can not say 明年，我学了中文.that sentence also indicate that you have finished the learning of chinese.