in practice

in reality (used to refer to what actually happens as opposed to what is meant or believed to happen)

實際上（用來指與預期或據信已發生的事相反的實際情況）

in theory this method is ideal—in practice it is unrealistic.

理論上這種方法很理想，——但實際上卻不現實。

out of practice

not currently proficient in a particular activity or skill though not having exercised or performed it for some time

不熟練，生疏

he was out of practice at interrogation.

他對審訊不太熟練。

practice makes perfect

used to convey that regular exercise of an activity or skill is the way to become proficient in it, especially when encouraging someone to persist in it

（尤用于鼓勵）熟能生巧