ought to的用法

(1) ought to表示出于义务、责任、道义等方面的要求而该做的事。如：

You ought to go to see the doctor. 你该去看病。

He said you ought to tell the police.

他说你应该去报警。

(2) ought to表示说话人的猜测。如：

John ought to know how to use the mchine.

约翰可能知道怎么用这个机器。

但要注意：在表示义务和猜测时，ought to通常可以和should换用。

need的用法

(1) 表必要性，多用于疑问句、否定句或否定意味的疑问句中。如：

You needn't hurry. 你不必着急。

Need I repeat it? 需要我重复吗？

(2) “need+完成式”表示对过去情况温和的责备、后悔等，通常限于否定句。如：

We needn't have hurried. 我们当时不必那么匆忙。

Depending on the sense, the verb “need” behaves sometimes like an auxiliary verb (such as can or may) and sometimes like a main verb (such as want or try). When used as a main verb, “need” agrees with its subject, takes “to” before the verb following it, and combines with “do” in questions, negations, and certain other constructions:

    He needs to go.

    Does he need to go so soon?

    He doesn't need to go.

When used as an auxiliary verb, need does not agree with its subject, does not take “to” before the verb following it, and does not combine with “do”:

    He needn't go.

    Need he go so soon?

The auxiliary forms of “need” are used primarily in present-tense questions, negations, and conditional clauses. Unlike can and may, auxiliary “need” has no form for the past tense like could and might.

When “need” is used as the main verb, it can be followed by a present participle, as in

The car needs washing.

(or by to be plus a past participle, as in)

The car needs to be washed.