Traditional English grammar is still not clear on how to define and classify subordinators for finite clauses in that grammarians mix up relative clauses, which are dependent on NPs, and subordinate clauses, which are dependent on other clauses. And different functions of the markers of subordinate clauses are not fully investigated.

The author of this aritcle found, through comparative studies, that relative clauses should be distinguished from subordinate clauses, which implies that words introducing relative clauses are not subordinators, and even among the markers of subordinate clauses some of them, such as before, because and if that indicates condition, are actually prepositions. There leaves a small class of subordinators for finite clauses, namely, that, whether and if that indicates interrogation.