yet 与already在现在完成时的用法

首先两者都是“已经”的意思
yet用于否定句和疑问句中(一般用在句末，但是也有用在句首的情况)
例如：I haven't done my homework,yet.
Have you done your homework yet?

already用于陈述句中（一般用于have/has的后边，动词的前边，但是也有用在句首或者句末的情况）
例如：I have already had breakfast.
I have been too Beijing already.

1.yet
yet常用于疑问句和否定句。在疑问句中，用来询问说话人期望发生的事情是否已经发生。在否定句中，表示说话人期望发生的事情尚未发生。例如：
John, have you finished that report yet? 约翰，你把报告已经写出来了吗？
John hasn't finished that report yet and I asked him for it three days ago. 约翰还没有写完报告，可我三天前就要他交了。

2.already
already表示“已经”，常用于肯定句中。例如：
It's only 11 o'clock and they have already had lunch. 现在才11点，他们已经吃过午饭了。
Are they building a new hotel here? There are too many hotels already. 他们要在这儿建一个新饭店吗？这儿的饭店已经够多了。

yet用于反问和疑问句，already用于陈诉句

U8-2:Have you read Treasure Island yet?知识点2现在完成时的结构与用法（1）（already，yet）

yet 只能放句尾
already哪都 行，