1. I think I've known this. 2. I have a new question. (There is no subordinate clause here.) 3. I think I've known this too. (Same as #1.) For #1, context is important. Normally, we would say "I knew that!", not "I think I knew that." "I think" means it is my opinion or I'm not sure. This phrase doesn't fit your context. I knew that. (knowing was completed in the past; not known from that time to now, when it has just been remembered or known again.) I have known that for a long time. (knowing started in the past and has been known from that time until now; never forgotten)