▲首先说明，高考原句以及你改动的两个句子，都正确：

1.  Alien had to call a taxi because the box was much too heavy to carry all the way home.

2.  Allen had to call a taxi because the box was much too heavy to be carried all the way home.

3.  Allen had to call a taxi because the box was much too heavy for her to carry all the way home.

▲以下是2010年山东卷高考题：That’s the new machine ______ parts are too small to be seen.

A. that         B. which       C. whose        D. what

在这个句子中，不定式用了被动式。但是，这句话中的不定式完全可以使用主动形式：

That’s the new machine whose parts are too small to see.

在“主语+ be + too+形容词+不定式”句式中，动词不定式一般表示结果。当something充当了句子的主语，跟不定式存在逻辑上的被动关系时，不定式既可以用主动形式，也可以用被动形式。这两者的区别是：

主动形式忽略了句子主语跟不定式的被动关系，而强调了人的主体行为，即人去做这件事，因此，可以在不定式之前加上逻辑主语for somebody。

被动形式强调了句子主语跟不定式的被动关系，在不定式之前不能加上逻辑主语for somebody，但可以在被动的不定式之后加上by somebody，以表示这个动作的执行者。例如：

第一组：

1. That’s the new machine whose parts are too small to see.

2. That’s the new machine whose parts are too small for the engineers to see.

3. That’s the new machine whose parts are too small to be seen by the engineers.

第二组：

1. The big stone is too heavy to lift.

2. The big stone is too heavy for a child to lift.

3. The big stone is too heavy to be lifted by a child.

在“主语+ be +形容词+enough+不定式”句式中，动词不定式也表示结果，同样有类似用法。例如：

第一组：

1. It looks as if the water isn’t clean enough to drink. 

2. It looks as if the water isn’t clean enough for the animals to drink. 

3. It looks as if the water isn’t clean enough to be drunk by the animals. 

第二组：

1. Are the apples ripe enough to eat?  

2. Are the apples ripe enough for the kids to eat?  

3. Are the apples ripe enough to be eaten by the kids? 

以上句子，在主动的不定式之前，并非一定要加for sb.，在被动的不定式之后，并非一定要加by sb. 这是理解问题的一种思路，帮助分清主动和被动之间的差异。

同意曾老师的分析。又如：

It is too hot to eat. 

= It is too hot to be eaten. 

= It is too hot for eating.

太烫了不能吃。

（摘自《薄冰英语惯用法词典》）