刘老师，向你请教form的主动和被动到底怎么样把握？
Collins online Dictionary 中针对form动词使用方法有一项讲解：When something natural forms or is formed, it begins to exist and develop. 同时提供了两个例句：
1. The stars must have formed 10 to 15 billion years ago. [VERB]
2. Huge ice sheets were formed. [be VERB|ed]
从讲解和例句中看，好像“自然形成”主动或被动两种使用方法并存，的不同之处吗？
但，牛津译林版初中英语教程中一段话需要改错： ...... Others like coal, oil and natural gas are dug up from the ground. They will form over thousands or even millions of years. 教参提供的正确答案为be formed. 说明此处只能用被动语态。
刘老师不了解到哪里了，我也很长时间没见他回帖。有点时间就说说我的怎么看。
自然形成的东西，没施动者或者说不了解施动者，确实有主动和被动两种使用方法，它们好像并无不同。但你说的这个改错题不是这么回事。
They will be formed over thousands or even millions of years.这句话的意思是煤，油和天然气形成的过程历程了thousands or even millions of years的时间，是一段时间，不是某个时间点，所以用被动适合。假如后面加个ago即over thousands or even millions of years ago，就是时间点了，那用主动被动都可以，意思跟前面的有不同。你举了两个柯林斯的例子，我也举词典中的几个例子，你可以对比看：
The solar system was formed 4.5 billion years ago.
The solar system formed 4.5 billion years ago.
A thick sheet of ice had formed over the water.
These rocks were formed over 4000 million years ago.
再举两个跟你的改错题类似的例子，没ago：
Beliefs about moral values and human nature are formed over a lifetime of experience.
Cultural identity is formed over hundreds of years.
需要说明的是，有的东西看着是自然形成的，而事实上有施动者，譬如钟乳石，施动者是含有矿物质的水不断下滴导致的。有时在这方面不怎么样分辨。
