1,2-addition occurs when the carbonyl oxygen (1) is attached by the electrophile and the carbonyl carbon (2) attaches to the nucleophile

for the 1,4 the 4 is the beta carbon

Not necessarily, it depends upon structure and size of molecule, usually free alcohols are more nucleophilic.

Hi I m confused that in simple furan why it do not show nucleophilic subtitution reaction. while in subtituted furan show it.

Williamson's synthesis an example of nucleophilic substitution rxn, in this rxn an alkyl halide is allowed to react with a Na alkokide.

A reaction in which a negative ion (nucleophile) attacks on a partially positive carbon atom then reaction is known as nucleophilic reaction, it may be substitution reaction or addition reaction.