A school dental service exists in theory, but in practice, there are few dentists to work in it.

学校牙科服务在理论上是存在的 但在实践中 很少有牙医从事此工作

In theory I'm on call day and night.

理论上，我不分昼夜随叫随到。

In theory, these machines should last for ten years or more.

从理论上讲，这些机器应能用十年以上。

In theory, the buyer could ask the seller to have a test carried out.

在理论上，买方可以要求卖方进行一次产品检测。

Achieving these goals is relatively easy in theory, yet quite difficult in practice.

实现这些目标从理论上说比较容易，但在实践上却相当困难。

In theory, you need molten iron in the core to generate one, but Mercury is so small, its core should have long since solidified.

理论上，你需要核心里的熔化铁来产生它，但是水星很小，它的核心凝固还要很久。