What is the difference between these two phrases: "I know it" and "I know this"? One explanation I found is that "it" means what you perceive and "this" is more abstract. Is it so?
It is not possible to say with any certainty what the nuanced differences might be. Here are some plausible uses:
She's taking money from the till, I know it! There should be more money here. We've had a busy day.
I know this (or This I know): when you're serious about going to medical school, you don't party every weekend.
For I know it, this is used more often in the past tense:
I knew it! He seemed strange from the beginning.
It is a confirmation of some suspicion or idea.
In the present tense it is pretty literal.
For I know this:
I can't believe that I could not repair the car. I know (how to do) this.
meaning you know how to repair the car but for some reason you couldn't. In this case it is an expectation of ability.