They will build it by january 和 They will have built it by January. 和 They will have built it until January. 和 They will build it until january 和 They will have been building it by(or until) January. 的差別在哪裡？如果不好說明，請提供一些例句。

“They will build it by January” and “they will have built it by January” mean the same thing.

“They will build it until January” is technically correct, but sounds unnatural. It is an odd use of the word “until”. You could say instead: “they will finish building it in January” or “they will build it by January”.

“They will have been building it by/until January” is unnatural as well. Depending on what you actually mean by the sentence, you could say any of these: “they will have started building it by January” or “they have been building it since January” or “they will be building it until January”