The description of a language at some point in time is a synchronic study and The description of a language as it changes through time is a diachronic study.

• 语言学是对语言的系统研究，对于一个学习英语的人来说，应该懂一点语言学的知识，它可以在理论上对学习语言有指导作用，有助于更好的学习语言，下面介绍一点语言学知识。