Can you tell me why we shouldn't separate "to" and "happen"? If that is an infinitive, I think for some splitting an infinitivc is accepted for some reasons.
Split infinitives are accepted by some and not by others. It is almost a matter of personal preference. My opinion follows, if you're interested.
Sometimes you have to split an infinitive because there is no other logical place to put an adverb, and some adverbs sound wrong (to some speakers) when placed before an infinitive. However, not is such a short adverb that it c