1. The correct sentence should be: "When she came late to the party, everyone was sitting around the fire."
2. No error.
3. No error.
4. The correct sentence should be: "The teacher of English encouraged her students to speak English as much as possible."
5. The correct sentence should be: "We lost the game, so we are all very bored now."
6. The correct sentence should be: "The operation involves putting a small tube into your heart."