以防

• lest should
• just in case

例句

1. 他们在窗户上加了窗条以防盗贼进入。
   They fitted bars to their windows to stop burglars getting in.
2. 可能下雨--你最好带把伞，以防万一（下起来）。
   It may rain you'd better take an umbrella (just) in case (it does).
3. 我已筹备了一笔准备基金以防不测。
   I have kept a reserve fund in case of accidents.
4. 我把一切东西都锁在保险箱里以防万一。
   I took the precaution of locking everything in the safe.
5. 我们应该使用无烟煤以防止空气污染。
   We should use anthracite to avoid air pollution.