The lecture on the history of UFOs was expected to be interesting. It turned out to be rather dull. (compound sentence)
The lecture on the history of UFOs was expected to be interesting, but it turned out to be rather dull.
The lecture on the history of UFOs was expected to be interesting. It turned out to be rather dull. (compound sentence)
The lecture on the history of UFOs was expected to be interesting, but it turned out to be rather dull.