BF3 has a planar triangular structure which arises from the sp2 hybrid orbitals.
(ground sytate) B 2s :

↿⇂

2p :

↿

(excited state) B 2s :

↿

2p :

↿

↿

These three sp2 hybrid orbitals are directed towards the corners of triangle and BF3 has a trigonal structure.

BH4– may be assumed to be made of a central B atom, 3H atoms and onehydride ion H–. In order to accommodate the 3H atoms and one H– ion, B undergoes sp3 hybridisation yielding four orbitals, 3 of which contain one e– each and one is empty. The fourth, empty orbital accomodates the H– ion. Thus, the structure of BH4– is tetrahedral.