The boy walk about in order to keep warm .（改为同义句）
The boy walk around in order to keep warm
There used to be low and dirty houses.(改为反义疑问句,didn't there）
Parents should allow teenagers to choose their own clothes .（改为含情态动词的被动语态）
teenagers should be allowed to choose their own clothes by parents
I don not know which mobil phone I should choose.（改为简单句）
I don not know which mobil phone to choose
In 1519 a traveler went to American from Europe .He discovered the tomato .（合并为含定语从句的复合句）
In 1519 a traveler went to American from Europe where he discovered the tomato.
I have been to Hong Kong __where___ _cars____ _go____ on the left.（同上）
保护环境是我们每个人的责任.It is __our___ ___duty__ _to___ ___protect__ the environment.
今天我离开家的时候卫生间的灯亮着.Tihs morning I left home __with__ ___the _ ___bathroom_ ___light_ __on__.IF someone takes away her books whitout her permission， she will ) him。（be angry with ；be in agreement with）when you leave the room ，don not forget to （ ） thelights。（turn off ：look……down）IF someone takes away her books whitout her permission， she will （be angry with ) him。（be angry with ；be in agreement with）when you leave the room ，don not forget to （ turn off ） thelights。（turn off ：look……down