在托福写作中，变换句式是提升文章表达力和避免重复的重要手段。以下是一些有效的句式变换方法：

变换主语

使用具体名词、抽象名词、动名词等作为主语，以增加句子的多样性和表达的丰富性。

例：

具体句：She attends school regularly.

抽象句：The regularity of her attendance at school is commendable.

变换句子的开头结构

使用名词、介词短语、副词、状语从句、分词短语、形容词等作为句子的开头，以增加句子的吸引力和表现力。

例：

名词短语：To begin with, even in such areas as computer science.

介词短语：In such areas as computer science, where it is estimated that much of what a student earns today will be obsolete in just five years.

简单句、并列句、复合句并用

巧妙搭配使用简单句、并列句和复合句，做到长中有短，短中有长，简单句、并列句和复合句样样俱全。

例：

简单句：She attends school regularly.

并列句：She attends school regularly and completes her homework on time.

复合句：She attends school regularly, which helps her maintain a good academic record.

非限定动词（短语）和与之作同样句子成分的其他成分互换

非限定动词（短语）可以做多种句子成分，可以代替各种从句，使句子更加简洁、成熟，避免过多使用从句所表现出来的呆板和单调。

例：

定语从句：The book written by him is very interesting.

非限定动词短语：Written by him, the book is very interesting.

主动语态与被动语态的转换

主动句型：动作执行者 + 动词原形 + 主语 + 其他成分。

被动句型：主语 + be动词 + 动词过去分词 + by + 动作执行者。

例：

主动句：They built a new bridge.

被动句：A new bridge was built by them.

陈述句型与疑问句型的转换

陈述句型：主语 + 谓语 + 宾语。

疑问句型：疑问词（如who, what, where, when, why, how）+ 谓语 + 宾语。

例：

陈述句：She reads books every night.

疑问句：Does she read books every night？

条件句型的转换

条件句型：如果A， 那么B。

让步句型：尽管A, B还是C。

例：

条件句：If you study hard, you will pass the exam.

让步句：Although you study hard, you may still fail the exam.

对比句型的转换

对比句型：A和B相比， A更怎样。

例：

对比句：She is taller than her sister.

强调句型的转换

强调句型：非常强调A。

例：

强调句：It was her kindness that made me feel warm.

倒装句型的转换

倒装句型：把谓语放在主语之前。

例：

正常句型：She is a teacher.

倒装句：Out came the teacher.

通过以上方法，你可以在托福写作中灵活变换句式，使文章更加生动、有趣且富有表现力。建议在实际写作中多加练习，逐渐掌握这些技巧。