请问一句英语 关于yet和still

Question

They have been planning to attend the show, but they _____ have not had the opportunity.

这里为什么不能用yet，只能用still呢？
谢谢！！

Answer 1

“yet”用于否定句。 放在句末
如：
I have not finished reading the book yet.

“still”既可用于肯定句又可用于否定句，表示动作或状态的持续。
如：
1) She still likes him.
2) I still can not understand you.
注意：“still”置于动词前后，会使句意发生变化。
如：
1) He is still standing. (continue to stand)
2) He is standing still. (=standing motionless)

Answer 2

yet还可以用于疑问句 否定 反问形式 still一般用于肯定句，除了仍然，还有竟然的强调意

Answer 3

因为yet用于否定句的句尾．如：I haven't have my supper yet.
still 用于句中．