
Michael S. answered 03/26/19
Experienced Teacher of English and Italian
Ciao! Great question! Are you familiar with Italian commands (gli imperativi)? The negative "tu" command for is expressed by non plus the infinitive. For example, if I tell you not to eat the ice cream (not sure why I would do that, but oh well!) I would say "NON MANGIARE IL GELATO!" as that is the negative tu command.
Now, let's look at your sample. If it were not a reflexive verb, we would say NON FERMARE MAI, however, it is a reflexive verb being used for the negative tu command, therefore, the "ti" becomes attached to the end. We cannot put it in the beginning because it is not followed by a conjugated verb leaving us with NON FERMARTI MAI.
I hope I did not make it too complicated. PLease feel free to follow up!