The phrases "till then" and "until then" usually mean the same thing. In a negative sentence, they mean that the thing won't be done until the time "then" comes. "I won't eat till/until 7 pm" or "I won't eat till/until then." It means that I will eat after 7 pm, but I won't eat before 7 pm. In a positive sentence, it means that the thing will be done continuously until that time. "I will sleep till/until 7 am" or "I will sleep until then." It means that I will continue to sleep until 7 am, and then I will probably stop. So if you see me at 6:30, I will be sleeping. If you see me at 3:30, I will be sleeping. So till then and until then are phrases for continuous actions. If I say, "I will not eat until 7 pm," then I will continue to not eat until 7 pm. If I say, "I will sleep until 7 am," then I will continue to sleep until 7 am. They are continuous actions (continually not eating or continually sleeping). The phrase "by then" means that the action will not be a continuous action. It will be an action for a specific moment. And it can be done at any time from now to "until then." "I will eat by 7 pm" or "I will eat by then." Here, I can eat any time from now until 7 pm for this sentence to be true. For example, right now it is 1:20 pm, and I say, "I will eat by 7 pm." If I eat at 4:30, then my sentence is true. If I eat at 6:42, then my sentence is true. If I eat at 2:58, then my sentence is true. Summary: "Till then" and "until then" are generally the same and describe continuous actions (either negative continuous or positive continuous actions). "By then" describes momentary actions done in a range of time (anytime between now and "then").