The word 下去in Chinese can be used in many situations with different meanings. Therefore, the foreign students should pay more attention to it when they study Chinese and put it into practical use. For instance, the meanings and usage of 下去in爬下去and请讲下去 are different even though they have the same position in the phrase. The former one is the basic application that implies the translocation from the high place to the low while the latter is the extensive application.

As matter of fact, there are several conditions about the exclusive use of 下去. The first one is to put after the verb to indicate the processing action will continue to move. For instance, 股票再这么跌下去，估计很多人都该睡不着觉了. Another condition that the students who learn Mandarin should know is after adjective, showing certain state that has appeared will continue like看着儿子一天天瘦下去，妈妈真是吃不下饭，睡不着觉.

When being placed after verb, it sometimes indicates to transfer from the high to the low or from the upstream authorities to the downstream. The speaker usually in the higher place like 请他尽快把任务布置下去. In addition, it can show the separation of the main objects and minor things. You can say 把玻璃上的剪纸撕下去吧.

However, those who study Mandarin online should know it is incorrect to say请你说话下去. Even though the extensive use of 下去after verb showing the meaning of continuing, the students should know the verb before 下去is not allowed to have objects. Accordingly, we should say请你说下去to instead of 请你说话下去.

In addition, when Chinese people say 下去, they usually hope the processing action will continue. Therefore, it often in the situation not happened yet. If it has happened, we seldom use下去. Therefore, it is wrong to say已经上课了，他还在玩下去.