it's a very nice f you to say so,i suppose i (could)为什么用coul

柯林斯英汉双解大辞典：常用短语-You can say "I suppose" when you want to express slight uncertainty。我想 (表示不太确定)[口语]例：I suppose I'd better do some homework。我想我最好做点家庭作业。例："Is that the right way up?"—"Yeah。I suppose so。" “那是上去的路吗?”— “是的。我想是。”其实也可以是can。用could更增加不确定的,谦逊的的语气。