Differentiating between verb+ては and verb+のでは?


According to the teaching materials I read, it appears that both verb+ては and
verb+のでは can refer to a condition leading to a negative result or the premise
of a negative comment. I’m not sure if this understanding is on the right
track. If it’s true, does it mean verb+ては and verb+のでは are often
interchangeable in a given sentence?

For example, can we say 年間200万円もかかる **のでは** とてもその大学には行けない。 and 年間200万円もかかっ
**ては** とてもその大学には行けない。 interchangeably?

If I get it wrong, what is the difference between verb+ては and verb+のでは? Does a
sentence exit in which verb+ては and verb+のでは are interchangeable but have
slightly different meanings?




When you use のでは, you need to choose the right tense. For example, you say
台風が来るのでは旅行に行けない referring to an approaching typhoon, but you say
台風が来たのでは旅行に行けない referring to a typhoon that has already come. And because of
the explanatory-の, it is usually used with a specific event that will almost
certainly happen (or has already happened) and affects someone's specific
decision at hand.

On the other hand, the ては version does not have tense distinction, and it
tends to sound like you are telling a general fact that is always true.
Proverbs such as
[腹が減っては戦は出来ぬ](https://jisho.org/word/%E8%85%B9%E3%81%8C%E6%B8%9B%E3%81%A3%E3%81%A6%E3%81%AF%E6%88%A6%E3%81%AF%E5%87%BA%E6%9D%A5%E3%81%AC)
and
[急いては事を仕損じる](https://jisho.org/word/%E6%80%A5%E3%81%84%E3%81%A6%E3%81%AF%E4%BA%8B%E3%82%92%E4%BB%95%E6%90%8D%E3%81%98%E3%82%8B)
use ては. Likewise, 台風が来ては旅行に行けない sounds to me like you're describing a general
fact (which may _also_ affect the current decision). In other words, 台風が来るのでは
may be closer to "Since a typhoon is coming" and 台風が来ては "When a typhoon
comes". (This does not apply to short "don't" type sentences such as
食べてはいけません, 見ちゃダメ and いなくなっちゃ嫌.)

In informal speech, のでは becomes んじゃ (e.g., 来るんじゃ, 来ないんじゃ), and ては becomes
ちゃ/じゃ (来ちゃ, 来なくちゃ).

