WH-questions through a kind of focus structure with the focus particle Ni: WH-questions with the NI copula. Both T and R can be questioned; note that when T is questioned, NI is deleted rather than stranded. Nb if subject is extracted/questioned, it must be resumed by a pronoun. (P. 985).
Source: Atoyebi et al. 2010: 159
Source: Atoyebi et al. 2010: 160