in the front of 是指某物(或某整体)内部的前部;
in front of指某物(或某整体)外部的前面.
也就是:一个是"内部",一个是"外部";一个是"前部",一个是"前面".
我相信你看了我下面列举的这个例子,就一定会明白了.
比如:The platform is in the front of the classroom.讲台在教室(内)的前部(讲台本身就在教室内部);
而另一种情况,The flowerbed is in front of the classroom.花坛在教室前面(花坛本身在教室外面).