Boron fluoride exists as BF3 but boron hydride doesn't exist as BH3. Give reason. In which form does it exist? Explain its structure.

Due to $\mathrm{p\pi }-\mathrm{p\pi }$ back bonding, the lone pair of electrons of F is donated to the B-atom. This delocalisation reduces the deficiency of electrons on B thereby increasing the stability of BF3 molecule.
Due to absence of lone pair of electrons on H-atom, this compensation does not occur in BH3. In other words, electron deficiency of B stays and hence to reduce its electron deficiency, BH3 dimerises to form B2H6.
In B2H6, four terminal hydrogen atoms and two boron atoms lie in one plane. Above and below this plane there are two bridging H-atoms. The four terminal B-H bonds are regular while the two bridge (B-H-B) bonds are three centre- two electron bonds.