-You've left the light on -Oh,sorry.I( )turn it off(be)

You've left the light on -Oh,sorry.I(am to)turn it off(be)
be to do意思是"就准备做。。。”“就去做。。。，”

I can't get into the room because I(have lost )my key to the door(lose)
have lost sth“已经丢了某物”。按句意，到现在还没找到，故用现在完成时。

1.
-You've left the light on
-Oh,sorry.I am to turn it off.
我正要去关灯。

be to do ...
即将要去做某事。
2.I can't get into the room
because I lost my key to the door.
我把钥匙落在房间里了。

用lost过去时，因为是过去“落在”房间里。

You've left the light on -Oh,sorry.I(am to )turn it off
意思是我现在就去把它关掉.

I can't get into the room because I(have lost)my key to the door(lose)
本来既可以用lost 也可以用have lost的
但是这句话的语境是我不能进入房间了,强调丢掉钥匙对现在造成的影响和结果.所以要用现在完成时.have lost