OK. I will follow the steps you suggested:
1. There is no electronegativity difference between the carbon and nitrogen, but there are non-bonding pairs on the nitrogen... it is a weak nucleophile.
2. BF3 has non-bonding pairs only on the Fluorine . Since the F atoms are more electronegative than the B atom, it is a pretty good nucleophile.
3. CH3SF has four non-bonding pairs on the Sulfor atom . Since there is no difference in electronegativity, I guess it is a pretty weak nucleophile.
4. (CH3)3 COH - There is a big difference between the electroneativity of carbon and oxygen and two non-bonding pairs on the oxygen, so it is indeed a nucleophile.
So, how can I determine between 1 and 3?
THanks !