[sentence A]。ところが、[sentence B]。
Note: ところが connects two sentences where B is an unexpected/contrary result of A. It typically starts a new sentence.
ところが is a conjunction meaning "however," "but then," or "even so." It is used to connect two sentences where the second sentence describes an unexpected result or outcome that contradicts what was anticipated from the first sentence. The key nuance of ところが is surprise – the speaker did not expect what actually happened.
ところが is placed at the beginning of the second clause to signal that reality turned out differently from what was expected or prepared for.
While しかし and けど simply express contrast, ところが emphasizes that the result was genuinely unexpected or surprising. It often implies that the speaker had a certain expectation or made preparations, only for things to turn out differently.
The clause after ところが describes what actually happened – an objective outcome. It should not be followed by the speaker's commands, requests, intentions, or suggestions. Because the unexpected result is something outside the speaker's control, sentences like ところが、~てください or ところが、~たほうがいい are unnatural.