if in doubt 

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

當(dāng)某人對(duì)某事不確定時(shí),，建議他做某事時(shí)就用這個(gè)短語,；翻譯為“如果不確定的話,，如果拿不定注意的話”

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

當(dāng)我去買東西的時(shí)候，我應(yīng)該鎖門,，還是讓門開著?

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

如果拿不準(zhǔn),，就把門鎖上。你總不希望有人偷你的東西吧,。