I bet idiom informal (also I'll bet)
（表示理解）有同感，我相信
said to show that you understand why someone has a particular opinion or feels a particular way
"I'm so annoyed with her." "I'll bet." “我实在太烦她了。”“我相信。”
"I was so relieved I didn't have to clean up after the party." "I bet you were." “晚会结束后我不必留下来收拾打扫，这让我大大松了口气。”“我知道你会这样。”