If (preposition)

If is an English preposition, as seen in If it's sunny tomorrow, '' we'll have a picnic.
As a preposition, if normally takes a clausal complement. That clause is, within the conditional construction, the condition on which the main clause is contingent. In such cases, if can be paraphrased as "in case" or "contingent on the case that".
If it's sunny tomorrow is a preposition phrase, and within a conditional construction it functions as an adjunct.
Where if takes a noun phrase or adjective phrase.
Traditional grammar books commonly treat if'', often understood as a single word encompassing both this preposition and the homonymous subordinator, as a "subordinating conjunction", a category covering a broad range of clause-connecting words.