I used to be a linguist till I become a writer.

过去我是个语言家, 后来成了作家。

He used to be a linguist till he turns writer.

他过去是个语言家, 后来成了作家。

Historical linguists have to identify and classify families of related languages in a genealogical family tree, and to reconstruct the protolanguage.

历史语言家们通过对各语系中语言进行甄别，勾画出家族谱系图，从而重新构建语系的原始语。