雅思作文中免不了要考虑句子多样性。在句子中常会有逻辑主语来开头。所谓的逻辑主语就是一些非谓语动词的真正主语。这些非谓语动词，譬如说动名词、不定式，还有现在分词和过去分词，很多同学都在用，但是很多时候忘记真正的主语。
　　1 不定式to do, in order to do something
　　In order to attract visitors, historic buildings should be maintained.
　　错误的原因：历史建筑是没有生命的，不可能有“in order to”
　　正确的句子：In order to attract visitors, city councils should maintain historic buildings.
　　2 when, while, before, after 等连词引导的现在分词
　　例句：When communicating with others, patience is important.
　　错误的原因：communicating这个分词的主语是人，不可能是patient
　　正确的句子：when communicating with others, one should show patience.
　　3 by+doing
　　例句：By setting rules, children can reduce behavioural problems.
　　错误的原因：setting rules的主语不是children
　　正确的句子：By setting rules, schools can reduce behavioural problems.