YOU SAID:

in order to open the door, a person who can change their shape has to cast magic in the water

INTO JAPANESE

扉を開くには、形を変えられる人が水に魔法をかけなければならない

BACK INTO ENGLISH

In order to open the door, someone who can change shape must cast water on it

INTO JAPANESE

ドアを開くには、形を変えることができる人が水をかけなければなりません

BACK INTO ENGLISH

To be able to open the door, someone who can change its shape must be watered

INTO JAPANESE

ドアを開けるには、形を変えることができる人に水をやらなければならない

BACK INTO ENGLISH

To open the door, you have to water someone who can change their shape

INTO JAPANESE

ドアを開くには、形を変えることができる人に水をやる必要があります

BACK INTO ENGLISH

To open the door you need to water someone who can change their shape

INTO JAPANESE

ドアを開けるには、形を変えることができる誰かに水をやる必要があります

BACK INTO ENGLISH

To open the door you need to water someone who can change their shape