In all of its uses, then is used when you want to talk about a point in time or sequence of events. If someone is asking when something happened, then is the appropriate word for your response.  For example, if your teacher asks you where you were at noon yesterday, you could respond, “I was at lunch then.” If someone asks when something will be ready, you could let them know to, “Come back tomorrow afternoon. I will have everything ready by then.” Another common use for then is to indicate sequential items. Use then to tell someone what comes next in time, space, or order. Some examples of these uses include:  We are going to leave at 9, and then we stop for lunch around 11. First, you line up part A and part B. Then, you screw them together. The inner planets go Mercury, Venus, Earth, and then Mars. Then can also be used to mean "in addition," "moreover," or "in that case." Use then when you need to add additional information to your sentence, or to modify outcomes based on conditions.  If you’re adding additional information you might say, “The dinner costs $20, and then you have to add the tip.” To express conditional information you may say, “If the weather is good, then we will go to the beach tomorrow.” In some special cases, then can be used as an adjective to indicate something that was true at the time, even if it isn't so anymore. You may hear then used this way often with people like politicians who once held a position, but no longer do.  For example, “That program was instituted in 2010 by then President Barack Obama.” This use isn’t limited to just people, though. You could also say something like, “The historian wrote about the then thriving state of Rome.”

TL;DR in Vietnamese:
Nhớ rằng then là một từ chỉ thời gian hoặc trình tự. Gắn kết một chuỗi các sự kiện với từ then. Dùng then để bổ sung thông tin hoặc điều kiện. Sử dụng then khi bạn muốn ngụ ý một điều gì đó là sự thật trong quá khứ.