[Sasuke]

I rarely bump into sentences which are like this:

They didn't tell me the truth to not make me sad.

I have ever thought that the correct one was ''not to make'' but I heard that both can be used... is there a rule or I can exchange them?

[susan53]

There is a old prescriptive rule which says you should never insert any words between to and an infinitive. But in practice people often do - remember Star Trek ... to boldly go where no man had gone before .

I'd suggest that if you're writing it can be more acceptable not to split the infinitive (like that!). But in speech you'll certainly hear both, and even in writing you'll often see it. There's no difference in meaning.