Traditional grammars typically have a category called the conjunction and distinguish between coordinating conjunctions and subordinating conjunctions. In point of fact, these two classes of words do not behave the same way at all, and so there is no good reason to think they are subtypes of a larger category. For that reason, we will treat these words as belonging to separate categories.
Coordinators are words that join grammatically equal units together. The principal coordinators are and, but, or, and nor.
Words whose function is to establish an unequal grammatical relationship, (e.g., that, for, to, whether, if).
(27) She asked me whether it was raining
Most subordinators can also function as other parts of speech: for and to can be prepositions, that can be a determiner, etc.), and so we will return to look at subordinators, and how to dintinguish them from other parts of speech, more closely in later chapters.