If (subordinator)

If is a subordinator similar to whether, marking the subordinate clause as interrogative (e.g., I don't know if that works).

As a subordinator, if has no conditional meaning (for that, see if (preposition)). Instead, it introduces subordinate closed interrogative clauses.^{: 972–973} 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 (whether/*if or not the room is ready), nor for a clausal subject (Whether/*If to attend was the question).

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.^{: 599–600, 738, 1011–1014}