already为肯定词，yet为非肯定词。

I have already seen it. 这是肯定语境，不能用yet非肯定词。

在问句中，用肯定词表示问话人期待一个肯定的回答。

Have you seen it already? 说话人认为你应该已经看到了，期待你的确认。

Have you seen it yet? 中性的提问，你可能看到了，也可能还没有看到。

这类似于下句：

Have you seen something inside? 说话人认为里面有东西，问你看到了没有。

Have you seen anything inside? 说话人并不知道里面是否有东西。

Do you want something to drink? 说话人准备好了给你喝点什么。

Do you want anything to drink? 说话人只是礼貌地问问，别当真。

already 和yet 的区别是什么? 

下面两句英文在意义上不知有何不同； 

(a)Have you seen it already?

 (b)Have you seen it yet?

【解答】照一般规则, yet用于问句, already用于陈叙句，如果把 already也用在问句中，往往有惊讶的意味。(b)例是普通问句的说法,是问对方是否已经看过,而(a)例则表示惊讶,为什么这样快就看过了。例如马戏班来此表演刚第一天见报,而你就已经看过了,真想不到。

代替yet用于问句或否定句中的 already，意思还是一样, 不过多表示一点惊讶而已,再举例句如下

Is he back already? ( already= thus early) (他这样早就回来了吗?)

Is it 10 o'clock already? (已经就有十点钟了吗?)

You’re not leaving us already, are you? (你这样早就要走了吗?)

再看 already正常用法的例:

The train has already left. (already= by this time) (火车已经开走了。)

When I called, he was already dressed. (already= by that time) (当我到他家时,他已经穿好衣服了。)

I've been there already, so I don't want to go again.

( already = previously; before now) (那里我已经去过,所以我不想再去了。)

至于yet 的普通用法,可分下列各种:

The work is not yet finished.

(在否定句中yet= up to now; so far)(那工作还没有完工。)

We have had no news from him yet. (我们到现在还没有接到他的消息。

Need you go yet? (你就一定要去了吗?)

Can’t you tell me yet? (yet=now)(你不能就告诉我吗?)

Have you learned to swim yet? (你已经学会了游泳吗?)

Be thankful you are yet alive.et=still) (你还活着,真是谢天谢地。)通常多用still。

I have yet (= still) more exciting news for you. (我还有好的消息报告你。)

It's the- best yet found. (到现在为止还没有见到比这更好的。)

The enemy may win yet (=at some future time; before all is over), if we relax our efforts. (如果我们松懈下来,敌人还是可以得胜的。)

As yet (=up to now or then) we have (had) not made any plans for the holidays.

(我们至今[或到那时为止还没有做出什么度假的计划。)

We shall not employ him nor yet his brother. (我们不雇用他,而且也不雇用他的兄弟。)

You must work yet (=even) harder. (你必须要再接再厉才行。)

I offered him still more, and yet he was not satisfied.

(我对他说可以再多给些,而他还是未能满足。

The logic seems sound, but yet it does not convince me.

(尽管说得有理,但我还是不能信服。)

already 和yet 的区别是什么? 下面两句英文在意义上不知有何不同； (a)Have you seen it already? (b)Have you seen it yet?

    【答】already 用来表示某事在早于预期的时间已经发生，或提前发生。如：Have you already finished? That was quick ! 你已经完成了？真快！(already 也可以放在句尾，表示强调。如：Are you here already ? 你已经在这里了？ )

      在疑问句中，yet 用来询问期望的事情有没有发生。如：

      Has the mail come yet ? 邮件送来了吗？

     （更多例句详见《牛津英语用法指南》第四版）