Boron fluoride exists as $BF_3$ but boron hydride doesn't exist as $BH_3$ . Give reason. In which form, does it exist ? Explain its structure. 

In $\mathrm{BF}_{3}$, lone pair of fluorine gives back support of electrons to boron atom by $p \pi-p \pi$ back bonding. This delocalization of electrons reduces the deficiency of electrons and thus reduces Lewis acidic character and increases stability of $\mathrm{BF}_{3}$.

In $\mathrm{BH}_{3}$, there is no lone pair of electrons on $\mathrm{H}$ atom, therefore, $\mathrm{BH}_{3}$ dimerizes to give $\mathrm{B}_{2} \mathrm{H}_{6}$. $4$ terminal $\mathrm{H}$ atoms and$2$ boron atoms lie in one plane and above and below the plane. Sothere are two bridging $\mathrm{H}$-atoms.