The skilful operation of a computer is hard to learn.

学会熟练地操作电脑是不容易的。

The frequency-centered SLA theory claims that input frequency is an essential factor affecting L2 acquisition order and determining the productability of language constructions.

摘要以频率为中心的二语理论认为，输入频率是影响第二语言顺序的重要因素，并对语言结构的能产决定作用。