The action of the unitary group associated with a quadratic extension of fields

Given a quadratic extension L/k of fields of characteristic different from 2 and a unitary space (V, f) of finite dimension over L, we give a representation, as simple as possible, of the form which f induces by restriction on a k-substructure of V. This, in turn, allows one to study the orbits of the unitary group U(V, f) in the set of k-substructures of V of a given dimension.