虽然冠词是一种虚词，没有词义，但是在某些习惯用语中，用还是不用冠词，用定冠词还是不定冠词，意义不同，有时甚至差别很大，使用时要特别注意。
下面这些词组就是很好的证据：
1. at table 在吃饭，进餐，用餐
They were at table when I entered.
我进去时他们正在用餐。
> at the table 在桌子旁
She sat at the table with pen and paper.
她拿着纸笔坐在桌旁。
2. in hospital 住院
He's in hospital, and in a confused state of mind.
他住院了，意识仍然不清。
> in the hospital 在医院里
He'll be in the hospital over Christmas.
圣诞期间他在医院。
3. by sea 乘船 (由海路)
Most of our luggage was transported by sea.
我们大部分行李都是海运的。
> by the sea 在海边
The children enjoyed playing by the sea.
孩子们喜欢在海边玩耍。
4. go to sea 当水手，出海，出航
My wish is to go to sea.
我的愿望是当海员。
> go to the sea 去海边
It was so warm a day that we decided to go to the sea.
那天天气非常暖和，所以我们就决定到海边去了。
5. in future 从今以后，将来
I asked her to be more careful in future
我要她以后要更加小心。
> in the future 未来
We expect better of you in the future
我们希望将来你有更大的成就。
6. on earth 究竟
What on earth had Harry done?
哈里究竟做了什么？
> on the earth 在地球上，在世上
The number of wild animals on the earth is dwindling.
地球上野生动物的数量正日渐减少。
7. go to school 上学
I usually go to school on foot.
我上学通常是走路去。
> go to the school 到学校去
He hurried off to go to the school.
他匆匆忙忙离开去那个学校了。