He knows what to do. He's been around
他知道该做什么。他阅历很丰富。
What difference will it make if he knows or not?
他知不知道有什么关系吗？
I can't help thinking he knows more than he has told us.
我总觉得他没把他知道的事全告诉我们。
He knows better than to judge by appearances.
他明白得很，绝不会凭表面现象来判断。
I'm still not totally convinced that he knows what he's doing.
我仍然不完全相信他明白自己在干什么。