It's bound to rain soon.

过一会儿肯定会下雨。

bound to: vt. 必然(不得不,必须,束缚于)，
例句：
I am bound to say I disagree with you on this point. 我必须声明不同意你这一点。
You are not legally bound to answer these questions. 在法律上，你没有义务非回答这些问题。