if in doubt

used when advising someone what to do if they are uncertain about something

当某人对某事不确定时，建议他做某事时就用这个短语;翻译为“如果不确定的话，如果拿不定注意的话”

A: When I go to the shop, should I lock the door, or leave it open?

当我去买东西的时候，我应该锁门，还是让门开着?

B: If in doubt, lock the door. You don't want somebody to burgle you.

如果拿不准，就把门锁上。你总不希望有人偷你的东西吧。