I must think twice before I can promise.

在我答应之前，我必须重新考虑一下。

《新英汉大辞典》

I can promise you one thing: whatever you choose to do for a career, if you work hard at it, eventually special things will happen.

我可以向你保证一件事：无论你选择什么职业，只要你努力工作，最终会有不一般的事情发生。

youdao

I can promise you, you'll have a wonderful time.

我保证你会玩得很痛快。

《牛津词典》

My best ideas are all in Berkshire Hathaway, I can promise you.

FORBES: Buffett Owns Wells Fargo in Personal Portfolio; Wouldn't Short Google and Apple

That I can promise you.

CNN: Blair's speech at 10 Downing Street - 1997 UK Elections

Now, these are some of the fights we've already had, and I can promise you there will be more fights ahead.

WHITEHOUSE: "White House to Main Street" Town Hall: Elyria, OH