1but 
/ˈbʌt, bət/ conjunction
/ˈbʌt, bət/ conjunction 1 — used to introduce a statement that adds something to a previous statement and usually contrasts with it in some way ▪ I don't know her, but my husband does. ▪ He wants to go to the movies, but I want to go to the museum. ▪ He plans to visit Boston and Chicago, but not New York. ▪ The book is not a biography at all but (instead is) a fictional account. ▪ It's not the music I don't like but (rather) the band themselves. ▪ She got the promotion not by luck but by hard work. ▪ The fighting has been going on for years. But to really understand the current situation, you have to look at the history of the region. ▪ She called his name, but he did not answer. ▪ He fell but (he) wasn't hurt/injured. ▪ I told him to stay, but he refused to. ▪ It might have been raining, but it was still a nice wedding. ▪ I'd love to come to the party, but [=however] I'll be away that weekend. ▪ I'm sorry, but I won't be able to help you. ▪ The dress is quite plain but (it's) pretty. ▪ They were polite, but not really friendly. ▪ Not only was it quite cold on our trip, but it rained the whole time too. ▪ I can't do it—but neither can you. 2 : other than : except
▪ We had no choice but to leave. ▪ They've done nothing but argue all afternoon. 3 — used in speech at the beginning of a sentence that expresses surprise, shock, etc. ▪ But you promised (that) you would help me with this. ▪ But it's not fair for them to treat him this way! ▪ They've arrived? But I thought they were coming Tuesday. 4 — used with a repeated word for emphasis ▪ Nobody but nobody could solve the riddle.
▪ We had no choice but to leave. ▪ They've done nothing but argue all afternoon.
but then 1 — used to introduce a statement that adds another and different thought to a preceding statement ▪ I'm surprised to hear that Tom has left the army. But then I suppose he never was the type to obey orders. ▪ I'm sure she would do a wonderful job on the project, but then I know she's very busy. — sometimes used to introduce an explanation for something ▪ His cakes are amazing—but then he did study with some of the best pastry chefs in the world. 2 — used to introduce a statement that tells about something different or surprising that happened next ▪ The team won the first two games, but then failed to win any of the next five. ▪ The disease was said to be untreatable, but then a new medication was introduced.
