如果你对某件事情 “have second thoughts”，就表示你 “不确定之前的想法是否正确，或经过重新考虑而改变主意”。注意，它和 “on second thoughts” 的意思有点不一样，后者一般用来表示你想更改刚刚做出的决定，并引出新的想法，意思是 “转念一想，再一想”。

例句
I was going to go for a swim in the sea, but now I know how cold the water is, I’ve had second thoughts.
我本来打算到海里去游泳，但是在知道水温有多冷之后，我改变了主意。

I was planning to buy the new smartphone, but after reading a couple of bad reviews online, I’m now having second thoughts.
我本来计划买新出的那部手机，但是在看到网上的几条差评之后，我现在得再考虑考虑。

I was going to have the pasta but on second thoughts, I’ll have a pizza – they look delicious!
我本来打算吃意面，但是再一想，我还是吃比萨饼吧，看起来太好吃了！