英语复合句变简单句的几种方法如下：

使用"too...to"或"enough to"结构

当句子中有"so...that"结构时，可以将复合句变为简单句，使用"too...to"或"enough to"来表达相应的意思。例如：

He is so young that he can't go to school. → He is too young to go to school.

He has so much money that he can buy a car. → He has enough money to buy a car.

将及物动词引导的宾语从句变为"及物动词+动词不定式（短语）"结构

将复合句中的宾语从句转换为"及物动词+动词不定式（短语）"的结构，从而将复合句变为简单句。例如：

He decided that he would buy a digital camera online. → He decided to buy a digital camera online.

We hoped that we would come back soon. → We hoped to come back soon.

使用"疑问词+不定式"结构

将疑问代词或疑问副词引导的宾语从句转换为"疑问词+动词不定式（短语）"的结构，从而将复合句变为简单句。例如：

I don't know which one I can buy. → I don't know which one to buy.

We wonder where we'll go this Sunday. → We wonder where to go this Sunday.

将宾语从句转换为相应的短语

将复合句中的宾语从句转换为相应的短语，从而将复合句变为简单句。例如：

We expected that you would come. → We expected you to come.

Now tell me what I should do. → Now tell me what to do.

将状语从句转换为状语短语

将复合句中的状语从句转换为状语短语，从而将复合句变为简单句。例如：

If it rains tomorrow, we will stay at home. → If it rains, we'll stay at home.

通过以上方法，可以将英语复合句有效地转换为简单句，使句子结构更加简洁明了。在实际应用中，可以根据句子的具体结构和语境选择合适的方法进行转换。