社会语义学认为，社会因素对语言单位意义的形成、变化以及语言单位的使用产生影响。

According to Socio semantics, social factors influence the formation, change and usage of the meaning of a language unit.

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在此基础上，本文还研究并分析了引起英语情态动词语义变化的因素，即动因，并对情态动词语义变化的主要机制进行了探讨。

Besides, this dissertation not only studies and analyzes the motivations that may cause semantic changes, but also probes into its mechanisms.

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