为便于理解和记住其用法与搭配，你的问题我们可以分以下几种情况来讨论：

▲表示“做某事是必要的”，英语用 It is necessary to do sth。如：

It is necessary to stamp your passport. 护照上必须盖章。

It will be necessary to proceed with caution. 小心进行是有必要的。

It may be necessary to make alternative arrangements. 或许有必要作备选的安排。

It’s often necessary to go over the same ground more than once. 常常有必要对同样的内容不只一次地进行检查。

▲表示“某人做某事是必要的”或“做某事对某人来说是必要的”，英语用 It is necessary for sb to do sth（不能用 sb is necessary to do sth）。如：

It was necessary for them to stay another day. 他们有必要再留一天。

It won’t be necessary for him to report to the police. 他没必要去报告警察。

Is it really necessary for me to attend the meeting? 我是不是真正有必要参加这个会？

Is it necessary for all of us to be present at the meeting this afternoon? 我们必须全体出席今天下午的会议吗？

※当然也可以用后接从句的形式（从句通常用 should+v，should 也可省略）。如：

It is necessary that he come to the office. 有必要让他到办公室来。

It is not necessary that everyone be a scientist. 没有必要使每一个人都成为科学家。

他有必要同我们一走去。

误：He’s necessary to go with us.

正：It’s necessary for him to go with us.

正：It’s necessary that he (should) go with us.

▲注意下面这类 Sth is necessary (for sb) to do sth 句子，此时的不定式为目的状语：

Getting enough sleep is necessary (for people) to stay healthy. 充足的睡眠是（人们）保持身体健康所必需的。

Experiment is necessary to find a treatment that suits you. 要想找出适合你的治疗方法得靠试验才行。

Entrepreneurship is necessary to answer the needs of national and international markets. 满足国内和国际市场的需要必须要有企业开拓精神。

※为便于理解，上面各句中的 (for sb) to do sth 也可放在句首：

(For people) to stay healthy Getting enough sleep is necessary.

To find a treatment that suits you experiment is necessary.

To answer the needs of national and international markets entrepreneurship is necessary.