Question

why nf3 is polar and bf3 is non polar

Answer 1

BF3 is non planer and also trigonal planer shape and is symmetrical with all respects. And no contain free unpaired electron because florien is very electro negative   dipole moment is 0.
In NF3 pyramid type structure and  florien is very electro negative so its resultant dipole is downward and dipole is not 0

Answer 2

Boron is a metal and nitrogen is a gas in uncombined state, BF3 bonds have no dipole moment whereas NF3 bond have dipole moment, atomic size of boron is smaller than that of nitrogen, BF3 is symmetrical molecule whereas NF3 is unsymmetrical. Because of this NF3 is polar and BF3 is non polar.