First, here's a material that made me keep thinking about this issue in the first place.
http://www.grammaring.com/as-if-as-though
- I feel as if I had known you all my life.
I haven't known you for that much, but I'm really comfortable with you like we are old friends.
- I feel as if I have known you all my life.
My one of old friends asked me how long we have known each other, and I'm guessing but I'm not sure whether when I was 3 or 4 that we first met. So, I'm saying like "I feel as if I have known you all my life."
I'd like to know the difference and use in exact situation. I'd really appreciate if you could help me.
Best Answer
Option 1 (using “had”) seems very unlikely to me. The situation it describes is that you currently feel that you previously experienced and previously completed knowing. It’s made even more unlikely by the reference frame “all my life”, which is not a previously completed period for anyone speaking!
Option 2 (using “have”) places the knowing in the continuous mood, meaning it started in the past (near beginning of the speaker’s life in this case) and continues through the present.
Option 2 is appropriate for both scenarios.
In the first case (conveying automatic comfort with someone), “feel as if I have” conveys the irrealis mood to the (untrue) fact of knowing this person your entire life.
In the second case (providing qualitative impression of relationship length), “feel as if I have” is again appropriate, because it is not strictly true that you have known this person your entire life. It feels that way because it is nearly that span of time (and because before you knew them you didn’t know much of anything else anyway), but irrealis is appropriate again (for a slightly different reason).