It's dangerous to play football in the street .

问题描述:

It's dangerous to play football in the street .
这句话等于It's in danger to play football in the street .

从语法上可以,但意思上不可以的.dangerous是“危险的”,in danger“处在危险之中”.
例如:Tigers are dangerous.老虎是危险的.(老虎很危险不能接近它,可能给你造成伤害)
Tigers are in danger.老虎处在危险之中.(老虎受到威胁,正处于频临灭绝之中)