I got these sentences here from WaniKani here, and I notice the de and wa most of the time together, so you read it as Dewa of course. But what is the usage of it in a sentence?
ここで泳いではいけません。
Do not swim here.
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In most cases では is just an emphasis on the local particle で.
This is a seperate case though.
In this case the で is part of the て-form of 泳ぐ.
this is the closest thing to a grammar entry i could find for it. If the comments are to be believed then it is basically like a gerund. (じゃ is just a shortened form of では that can be found in most [but not all] dialects)
http://www.jgram.org/pages/viewOne.php?tagE=teha
Edit: found out it also has an article on tae kim’s so here you go:
http://www.guidetojapanese.org/learn/grammar/must
As always leebo is right.
Te form is still similar to the gerund but can only be used with a は in a must not context and with the three words mentioned on tae kim’s.
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Yeah, ～(て-form)は[negative word] is a grammar point for “you mustn’t [something]”
The negative word can be いけない or だめ or some others as well.
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Ah oke, that makes sense.
(Thought for a moment you were asking about 出羽. That’s a former province in the Tohoku region which covered most of Yamagata and Akita Prefectures. 
)
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