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4. Do you mind? は もんだいありますか
Opening the windows は まどを開けることが
Me is often put before the situation of opening the windows.
My means my action of ....
Do you mind Me/my Opening the windows?
5. It is は subject が 分かりません 時 situation 本当 subject 成ります
Situation = dangerous. It is dangerous
To swim or swimming can either one be used.
Swimming is the act to swim
どこ？ in this river
It is dangerous to swim/ it is dangerous swimming in this river.
Native American speakers usually say to swim in this case though.
ご丁寧にありがとうございます！