"[1] Most dictionaries and many traditional grammar books use the term subordinating conjunction and include a much larger set of words, most of them prepositions such as before, when, and though that take clausal complements.
It is often possible to substitute if for whether, the main exceptions being when the subordinate clause functions as the subject, as in Whether it's true is an empirical question and cases with or not, such as I'll be there whether you are there or not.
[2]: 655–656, 1321–1322 To is arguably a subordinator when it marks infinitival verb phrases such as To be sure, we'd have to double check (but a preposition in I went to Peoria).
[2]: 569, 907–909 Various linguists, including Geoffrey K. Pullum, Paul Postal and Richard Hudson, and Robert Fiengo have suggested that to in cases like I want to go is an acutely defective auxiliary verb: one with no tensed forms.
[4] Rodney Huddleston argues against this position in The Cambridge Grammar of the English Language, but Robert Levine counters these arguments.
[6] Traditional grammar includes in its class of "subordinating conjunctions" prepositions like because, while, and unless, which take a clausal complement.
But since at least Otto Jespersen (see English prepositions for the historical development of the idea) most modern grammarians distinguish these two categories based on whether they add meaning to the sentence or are purely functional.