Question

how the NH3 donate electron to BF3,while the BF3 have a complete octet???​

Answer 1

Hi,
See BF3 does not have a complete octet, the covalent bonds have been formed between boron and fluorine,

Boron shares it's each of the three electrons with other three electrons each one from the fluorine atoms.

So, it's obviously electron deficienct
and also, in chemistry we call these type of compounds as Lewis acids, which accept pair of electrons from other electron rich species.

And here, we can even see that NH3 is an electron rich specie, because Nitrogen having 5 electron in it's outermost valence shell, shares only 3 of it's electrons to form covalent bonds with 3 other hydrogen atoms.

Thus, it still has 2 electrons left as a lone pair on itself.

So, if we someone obviously needs your help, you should help him/her.

That is what, the good boy , NH3 does here, to help out The already Electron deficienct BF3

Thus, we call NH3 as a Lewis base, because it can donate it's pair of lone electrons.
And thus actually gives away both of it's electrons to form something what is called a coordinate or dative bond.

That's it !
Hope this helps you !

Regards