Teaching a game without ruining the game
The most common way to kill an evening is a thorough and complete explanation.
The instinct when teaching is to be thorough. You know the game, you know where the confusions lie, and it feels responsible to head them all off in advance. So you deliver the full rules, every exception, every edge case, in a twenty minute monologue to people whose attention departed around minute four. By the time anyone touches a piece they are already behind and slightly embarrassed, which is precisely the state in which a person decides they do not like this sort of thing and would rather do something else next time.
The better approach is uncomfortable for the teacher because it involves being wrong on purpose. Explain the goal, explain what a turn looks like, and start. Let the edge cases arrive when they arrive. Allow the first round to be a bit incorrect, and say so cheerfully, and take back a move where taking it back is kinder. New players learn a game by taking turns and watching what happens, not by receiving information in advance, and every minute spent front loading the rules is simply a minute of actual learning postponed.
It also matters that the teacher not win the first game too comfortably. This is delicate: nobody wants obvious charity, and playing badly on purpose is easily detected and mildly insulting. But a teacher can choose the interesting line over the ruthless one, can flag a mistake before it becomes fatal, and can decline the crushing play that ends the game early and teaches nothing. The aim of a first session is not a fair or accurate result, and treating it as one misreads the job entirely. The aim is that everybody finishes the evening wanting a second session.