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.
I have heard that 了 can serve to indicate actions to be completed in the future, if the action is to be completed by the time of some other action you also talk about. I see this in several grammar books. And as an on-line source.
has several examples such as
你 到 了 告诉 我。future action Nǐ dào le gàosu wǒ. When you have arrived, tell me.
The comment by user6065 says that "明年，我学了中文" is wrong because it does not give any second action after the studying of Chinese. But 了 can be used correctly to indicate a completed action that has yet to come.
'I would have learned ...' means 'I did not learn ..' it is subjunctive. Something got in the way.
'I will learn ...' means 'I have not yet learned ...' and you are looking to the future.
So why is it that I cannot say
to indicate that?
- You are talking about the past
First I studied Russian.
The next year I studied Chinese.
I was studying Chinese and getting good.
by the next year I would have learned Chinese, but then the war started.
- You are talking about the future
“By this time next year, I'm sure I will have learned Chinese.” （wish I could say that!!)
明年 is most definitely future. You need 将。
"了" is usually used in describing something happened in the past. If you would like to say something in the future with "了," there should be the other action to express that after you finish action one, and then you are going to make the action two.
For example, "明年我学了中文以后,我还要学法文" (After I learn Chinese in next year, I'm going to learn French, too.)
"了" means something finish or is about to be finished. Can't use it in further tense. It is one words with two pronunciation and similar meaning.
A △了 liao第三声
- 懂，明白(understand) e.g了解，一目了然
- 完结，结束(finish) e.g不了了之、没完没了
- 完全(complete) e.g了无愧色、了不相干
- 在动词之后，与“得”“不”连用，表示可能或不可能(“得了” “不了” indicate something possible or impossible) e.g干得了、办不了
B △了 le轻声
- 用在动词或形容词后面表示动作或变化已经完成或将要完成(Behind the verb or adjective, indicate the action or change finished or is about to be finished) e.g. 我喝了一瓶水（I have drunk a bottle of water ）
- 用在句子末，表示肯定语气(It is used at the end of the sentence,indicate something you are sure) e.g. 刮风了、可以走了
- 表示感叹语气(sigh) e.g太棒了(great!)、这下好了(thats right!)，有救了
So,back to the question
Q: 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?
A: In the sentese," 了" following a verb so it should pronounce "le轻声" . Because of matching B1 meaning “The action or change finished or have done” If 明年and学了use in the sentence, it will be a grammar mistake. Like:I have learned Chinese next year. 明年，我学了中文
So uncertain,have I learned it?Or I am going to learn it?