necessary

1.下面每组里的第一句比第二句普通得多：

• It is necessary to finish the work within this week.
  The work is necessary to be finished within this week.
• It is necessary for us to finish the work within this week.
  The work is necessary to be finished by us within this week

注意不可说This work is necessary to finish within the week，也不可说We are necessary to finish the work within this week。

2.necessary后面接for或to没有区别，如下面各句都可以说：

• Food is necessary for（或to）life.
• Books are necessary for（或to）students.

但It is necessary for us to do this等句子里的for不可改作to。

3.It is（或was）necessary that...里该用should或虚拟现在式。例如：

• It is necessary that he should come here every Sunday.
• It is necessary that he come（不用comes）here every Sunday.
• It was necessary that he should come here every Sunday.
• It was necessary that he come（不用came）here every Sunday.

4.说“必要的东西”用necessary或用necessity都可以，如the necessaries of life和the necessities of life，Do you consider a motor-car a necessary? 和Do you consider a motor-car a necessity?在用复数形式的场合，似乎在英国多用necessaries，在美国多用necessities。说“必要的事”通常用necessity，不用necessary，如It has now become a necessity to read the whole document again里不用necessary。