Donald Trump has very little regard for the truth. He has told an enormous amount of lies since assuming office. When the New York Times printed a list of his lies, his supporters claimed that all presidents lie and Trump hasn't lied more than other presidents (especially President Obama). The paper investigated that -- and found it simply was not true. Trump has lied far more than Obama.
From the New York Times:
We applied the same conservative standard to Obama and Trump, counting only demonstrably and substantially false statements. The result: Trump is unlike any other modern president. He seems virtually indifferent to reality, often saying whatever helps him make the case he’s trying to make. In his first 10 months in office, he has told 103 separate untruths, many of them repeatedly. Obama told 18 over his entire eight-year tenure. That’s an average of about two a year for Obama and about 124 a year for Trump.