不定

将来的情况不定，我们只能做好准备，应对各种变化。

The future is uncertain, so all we can do is prepare for different possible changes.

出发的时间还不定，我们要等通知以后再安排行程。

The departure time is still uncertain, so we will arrange the trip after we receive further notice.

他晚上不定几点才能到，我们还是先吃饭吧。

There is no telling what time he will arrive tonight, so we should eat first.

这场雨不定什么时候会下，出门最好带上雨伞。

There is no telling when it will rain, so it is best to take an umbrella when you go out.