What syntactic function does the word "if" serves in this sentence.
Certain pesticides can become ineffective if used repeatedly in the same place.
This sentence is taken from GMAC prep material so I am assuming that the sentence is grammatically correct.
According to oxford dictionary (https://www.lexico.com/en/definition/if) the word if can only be a subordinate conjunction.However, there is no subject after if in the sentence above.
Moreover the first example sentence given by oxford dictionary for word if is
if you have a complaint, write to the director.
but there is no main clause in this sentence.
So can the word if serve some other syntactic function also?