ところが

ところが can be used at the beginning of a sentence or phrase to express that something unexpected based on the previous context has occurred. This occurrence must be beyond the speaker's control. It's often translated as "but" or "however."

先生のオフィスアワーに研究室に行った。ところが、先生はいらっしゃらなかった。
I went to sensei's office during office hours, but he wasn't there.

昨日、アザにコンビにで商品をたくさん買っていた。ところが、家に行った時、いなかったんだ。
Yesterday I bought a bunch of goods for Aza at the konbini. But when I went to her house, she wasn't there.

試験のために徹夜をするつもりでコーヒーをたくさん飲んだ。ところが、すぐ寝てしまった。
I drank a lot of coffee so I could stay up all night for this exam. But I fell asleep quickly.

もちが大好きなの。ところが、リーさんが作るもちは嫌いんだよ。
I really like mochi. But I hate the mochi that Lee-san made.