英语"现在完成时"小疑问

想问下有哪些词，或者词组可以判定一定要用现在完成时。
yet already since

还有 Have you seen him yet?是什么意思？
你见过他吗？

像句尾有 yet already since (before)
for + 一段时间
eg:I have seen you before, but I can't remember where.
我以前见过你，但是不记得在哪里了。

Have you seen him yet?
你以前见过他吗？