Trump lies, everyone knows that. What seems to be the more difficult thing to convince others is that he lies at a rate and magnitude that is absurdly higher than anyone else.
For clarity, let’s look at what a “pathological liar” is. I’ll let you take a moment to go read The Wikipedia page.
Trump’s former Press Secretary, Stephanie Grisham, recently attested to his disregard to the truth at the DNC:
He has no empathy, no morals, and no fidelity to the truth