這個句子正確嗎?If i know something that I cannot tell anyone

題目：

這個句子正確嗎?If i know something that I cannot tell anyone
If i know something that I cannot tell anyone 這句里用know還是用knew?
爲什麼？這個不是假設句子嗎？

解答：

If i know something that I cannot tell anyone
這不是假設句
是如果我知道.
如果是假設句,那就
if i had known sth .
假如我知道.