Why 忘れてる in「来週（明日）には忘れてる」?


> 来週（明日）には忘れてる

Initially, this strikes me as odd, because I would expect 来週には忘れる. But it
seems to make sense: " _Come next week, (I) will have forgotten about it._ "
How would 忘れる work differently in such sentences? (random sentences extracted
from the web)

> 多分来週には忘れてるだろうな
>
> たぶん来週には忘れてるけど！
>
> どうせ明日には忘れてるだろうし友情的な意味で言っただけだろうな

Also, it seems to me は is indispensable here. Is that correct? Why?




I think the main difference lies in when the said action (instant-state-change
verb, 忘れる in your case) can happen. 来週には○○する implies something will happen
around or (shortly) before 来週 **but not now** , whereas 来週には○○している implies it
can happen **anywhere** between now and 来週.

[![図](https://i.stack.imgur.com/AFHUR.png)](https://i.stack.imgur.com/AFHUR.png)

In your situation, the difference between 来週には忘れる and 来週には忘れている is not very
important. But compare the following sentences:

  * 私は500年後には死ぬ。  
(The speaker is not an ordinary human because this sounds like he intends to
live at least for another 300-400 years. 私は500年後に死ぬ without は means he will
die strictly in 500 years, and you even need an ability to precisely predict
your future to say this.)

  * 私は500年後には死んでいる。  
(An ordinary human can safely say this. は is usually necessary, but it can be
omitted in a relative clause. 2年後に死んでいる人 and 2年後には死んでいる人 are interchangeable.)

  * 10年後には結婚したい。  
(He probably doesn't want to get married right now.)

  * 10年後には結婚していたい。  
(He may want to get married even now, and 10年後 is a worst scenario.)




I think the main difference lies in when the said action (instant-state-change
verb, 忘れる in your case) can happen. 来週には○○する implies something will happen
around or (shortly) before 来週 **but not now** , whereas 来週には○○している implies it
can happen **anywhere** between now and 来週.

[![図](https://i.stack.imgur.com/AFHUR.png)](https://i.stack.imgur.com/AFHUR.png)

In your situation, the difference between 来週には忘れる and 来週には忘れている is not very
important. But compare the following sentences:

  * 私は500年後には死ぬ。  
(The speaker is not an ordinary human because this sounds like he intends to
live at least for another 300-400 years. 私は500年後に死ぬ without は means he will
die strictly in 500 years, and you even need an ability to precisely predict
your future to say this.)

  * 私は500年後には死んでいる。  
(An ordinary human can safely say this. は is usually necessary, but it can be
omitted in a relative clause. 2年後に死んでいる人 and 2年後には死んでいる人 are interchangeable.)

  * 10年後には結婚したい。  
(He probably doesn't want to get married right now.)

  * 10年後には結婚していたい。  
(He may want to get married even now, and 10年後 is a worst scenario.)

