6.作补足语
有宾语补足语和主语补足语两种。
1)作宾语补足语
有一些及物动词除跟宾语外，还需要有一个成分作逻辑上的补充说明，使得宾语的状况或行为或特点等的意思完整。这个成分称为补足语。因为说明宾语，因此称宾语补足语。不定式可作宾语补足语。如：
Many parents allow their children to make their own decisions.
很多父母允许他们的孩子自己拿主意。
The trainer made the elephant dance before audience.
训兽中使大象在观众前跳舞。
注：
① 一般需要带to不定式作宾语补足语的动词有：advise, allow, ask, cause, consider, expect, find, imagine, know, tell, think, want, wish等www.ＥxamdＡ.CＯM
② 需要不带to的不定式作宾语补足语的动词有：feel, have, hear, let, make, notice, see, watch, listen to, look at
③ help后面的作宾语补足语的不定式带to或不带to都可以。
help Mary wash her clothes
help Mary to wash her clothes
2)作主语补足语
带有宾语补足语的谓语动词变为被动语态时，宾语成了主语，宾语补足语也就相应地成为主语补足语。当动词不定式用作主语补足语时，一律要用带to的形式。如：
Their children are allowed to make their own decisions.
允许他们的孩子自己拿主意。
The elephant was made to dance before audience.
使大象在观众前跳舞。
不定式结构在英语文章中用频率较高, 如下面的这段文字中, 不定式结构出现了5次, 请大伙确认这类不定式结构在句子中的语法功能.
So far there is no data to suggest that people living on metal-rich soils experience a potential health hazard1. They are also the most likely ones to eat potentially dangerous metal-rich soil. Some trace2 metals are alleged3 to cause cancer and also known to cause poisoning中毒. There is a vital need to understand the potential risks and long-term health effects of living on naturally contaminated4 soils.
提示: 相邻语句总是围绕同一中心进步, 在词汇上常常表现为: 相邻语句中出现呼应的词汇.
剖析：第1，2，4句中不定式是作定语;第3句的不定式是作主补;
Ⅲ不定式的逻辑主语
如要表明不定式或不定式的动作发出者。如：
Is the road wide enough for two cars to pass?
这条道路是不是宽得足以使两辆车通过?
The conference is too important for you to miss.
这会议太要紧了，你不可以错过。
