文法 Grammar Nakereba Narimasen/nakya

 返さなければなりませんkaesa nakereba narimasen 

I have to return it

Example 返す　kaesu return

Negative 返さない　not to return 

Replace the nai part with nakereba narimasen so

It means I have to return  

Another casual form of  nakerebanarimasen is nakya 

返さなきゃ　which is the casual  form .

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https://www.nhk.or.jp/lesson/english/teacher/36.html