上面三个句型都是: 阻止某人、某物做某事或阻止某人、某物被.....。 但它们的用法还是有点不同：

在句型stop sb./sth. from doing sth. /being done和 prevent sb./sth. from doing sth./being done中，主动语态中from可以省去，而被动语态中from不可以省去。

句型keep sb./sth. from doing sth./being done，在主动和被动语态中from都不可以省去。

如: We should try to stop the trees( from )being cutting down.

我们应该想方设法阻止树被砍伐。

They managed to keep the accident from happening .

他们成功地阻止了事故的发生。

2) free ... from... 使解除, 使摆脱 如：

They aim to free the students from too much homework.

他们的目的是使学生摆脱繁重的家庭作业。

3) be free of sb./sth. 摆脱某人/某物

I'm free of that job at last. 我最终摆脱了那项工作。

　　 [即学即练5] You must take care to keep the dirt.

　　　　　　　　　你一定要当心，不让伤口受感染。

八．自主探究

难句分析

1.【原句】 Crops such as peas or soybeans put important minerals back into the soil, making it ready for crops such as wheat or corn that need rich and fertile soil. 像豌豆和大豆这样的农作物把重要的矿物质带回土壤，从而使之适宜于种植要求土壤肥沃的农作物，比如玉米和小麦。

　　[分析]

1）本句结构比较复杂。其主干结构为: 主句(Crops put important minerals back into the soil) + 状语(making it ready for crops...)

　　2）状语中又包含"make + it + 形容词"结构和由that引导的定语从句(that need rich and fertile soil)。

3）making it ready for crops... 在句中作结果状语。

2. Some organic farmers prefer planting grass between crops to prevent wind or water from carrying away the soil, and then leaving it in the ground to become a natural fertilizer for the next year's crop.

　　 [分析]

1）本句的主干结构是一个简单句, 其结构是: 主语(farmers) + 谓语(prefer) + 并列宾语(planting ... + leaving ...)。

2）其中between crops和in the ground作地点状语。