1. Used to express obligation or duty:
You should send her a note.
2. Used to express probability or expectation:
They should arrive at noon.
3. Used to express conditionality or contingency:
If she should fall, then so would I.
4. Used to moderate the directness or bluntness of a statement:
I should think he would like to go.
5. Like the rules governing the use of shall and will on which they are based, the traditional rules governing the use of should and would are largely ignored in modern American practice. Either should or would can now be used in the first person to express conditional futurity:
就象作为shall 和 will 词的基础的限用的用法规则一样， 适用于should 和 would 这个词的传统使用规则在现代美国英语中也已被忽略了。 现在should 或 would 这两个词中的任何一个都可以用于第一人称，表示条件式中的将来：
If I had known that, I would (or somewhat more formally, should ) have answered differently.
如果已经知道了这个情况的话，我就（或正规一点用 should ） 不会那么回答了 ，
6. But in the second and third persons only would is used:
If he had known that, he would (not should ) have answered differently.
如果他知道那个情况的话，就（不能用 should ） 不会那么回答了。
7. Wouldcannot always be substituted for should, however. Should is used in all three persons in a conditional clause:
但是Would并不是总是能由 should 代替。 Should 在三种人称的条件从句中都可以用：
if I (or you or he ) should decide to go.
如果我（或 你 或 他 ） 决定要去。
8. Should is also used in all three persons to express duty or obligation (the equivalent of ought to ):
Should用于这三种人称的表示职责和义务的句子中（相当于 ought to ）：
I (or you or he ) should go.
我（或者 你 或者 他 ） 应该去 。
9. On the other hand, would is used to express volition or promise:
I agreed that I would do it.
10. Either would or should is possible as an auxiliary with like, be inclined, be glad, prefer, and related verbs:
而would 或 should 都可以作助词和 like，be inclined， be glad，prefer 及其相关词语一起使用：
I would (or should ) like to call your attention to an oversight.
我想（或 should ） 请你注意一下一个疏漏之处 。
11. Here would was acceptable on all levels to a large majority of the Usage Panel in an earlier survey and is more common in American usage than should. · Should have is sometimes incorrectly written should of by writers who have mistaken the source of the spoken contraction should've. See Usage Note at if, rather, shall
在此处，在一次早期的调查中对大多数各阶层的使用者来说，would 是可以接受的， 且在美国用法中比should 更为常见。 有时书写者把should have 误拼成 should of ， 因为他们把口头缩略形式should've的来源给弄错了 参见 if， rather， shall