If (subordinator)
If is a subordinator similar to whether, marking the subordinate clause as interrogative.
As a subordinator, if has no conditional meaning. Instead, it introduces subordinate closed interrogative clauses. This aligns if with whether, and the two may often be used interchangeably, as in I doubt whether/''if that's true. However, if is more constrained. As examples, it can appear neither in the whether or not construction, nor for a clausal subject.
Traditional grammar books commonly treat if'', often understood as a single word encompassing both this subordinator and the homonymous preposition, as a "subordinating conjunction", a category covering a broad range of clause-connecting words.
History
The Oxford English Dictionary traces the word back to its Germanic roots, with cognates in several old Germanic languages, each broadly carrying the meaning of 'if' or 'whether'. The subordinator if existed in the earliest records of English. Examples of the subordinator follow:The OED notes the existence of forms with an initial g, reflecting a palatal /j/.