I was really sure that our lesson started at 9:30.
ちょっと難しいですね。 “start” は例えば “What time does our lesson start tomorrow?” に使います。 答えの “It starts at 9:00.” では “starts” です。解答2はすごく思い込んでいた時に使います。他に “I could have sworn our lesson started at 9:30.” の言い方もあります。 “I’m sorry I got the time mixed up.” も一緒に使えます。