on guard
watchful警惕；警戒
The police warned people to be on guard for pickpockets during the Christmas rush．警察警告人们在圣诞购物高潮时谨防小偷。
Be always on guard against complacency．要时时警惕自满情绪。
on hand
1．nearby；within reach附近；在手头
Do you happen to have any aspirin on hand？I seem to be getting a headache．你身边有阿斯匹林吗？我似乎头痛。
Alwaya have your dictionary on hand when you study．学习时要随时将词典放在手边。
2．present出席；在场
You'd better wait here so you'll be on hand when you're needed．你最好在这儿等着，以便需要你的时候你在场。
Mr．Baker's secretary is always on hand when he appears in public．贝克先生出现在公众面前时，他的秘书总是在场。
3．in stock；available现有（存货）
We have some new woollen goods on hand．我们现有一些新羊毛织品。
The department store has a large stock of goods on hand．这家百货商店现有大量存货。