Which is most grammatically correct I decided to not do that or I decided not to do that?

Answer 1

I would say "I decided not to do that". It seems more correct. I hope that helps.

Conservative editors would disallow "to not do" as a split infinitive. But since infinitives often occur without the infinitive marker to, as in "He can't see," we may argue that there really is no such thing as a split infinitive. That being the case, there may be a perceptible difference in meaning between "to not do" and "not to do," as in the following examples, with emphasis indicated by upper case letters: I decided not to DO it; I decided to NOT do it. Putting the negative closer to its verb seems to recall that ancient feature of English, negation of a verb by the prefix n-, as in the Old English ic neom wyrthemeaning "I am not worthy."