Why NF3 is a weak base than NH3?
F(fluorine) is the most electronegative element in periodic table. It has the highest tendency to gain electron (it has more tendency then hydrogen to gain electron ). So the 3 F atoms in NF3 attracts the lone pair of electrons in N and hence it becomes difficult for N to loose that pair of electrons, and therefore acts as a weak base. But in NH3, Hydrogen is less electronegative then Fluorine, and has less tendency to attract shared pair of electrons. So, in NH3, the lone pair of electrons in N are weakly attracted by H atoms, and hence N can loose those shared pair of electrons (if needed).
Or I can ans. your ques. in short as "F is more electronegative than H and has more tendency to attract shared pair of electrons"
I hope it will help.