ところが
But then (a result contrary to expectation)machine translation
FormationAt the start of the following sentence: Sentence A。ところが、sentence B。
ところが opens a sentence that reports a result CONTRARY to what the previous sentence led you to expect: 晴れると思っていた。ところが、急に雨が降り出した (I thought it would be sunny. But then it suddenly started pouring). The focus is on surprise — the speaker did not see that development coming.
Because it reports a fact that already happened, the clause after ところが is always something real, mostly in the past tense; do not put a command, an invitation, or an intention after it. This is what sets it apart from しかし — the neutral conjunction "but," usable for both arguments and opinions.
Watch out for its easily confused sibling: ところで (the topic-changing "by the way") — only the final character differs, yet the job is completely different. Memory tip: が is contrastive, so ところが means "but then"; ところで turns to another subject.
Examples
晴れると思っていました。ところが、急に雨が降り出しました。
I thought it would be sunny. But then it suddenly started pouring.
兄に相談しました。ところが、全然聞いてくれませんでした。
I went to my older brother for advice. But he wouldn't listen at all.
簡単だと思って始めました。ところが、意外に難しかったです。
I started it thinking it would be easy. But it turned out to be unexpectedly hard.