"on" is used with some nouns or adjectives to say who or what is affected by sth.

X an impression on Y is a set phrase...it's a grammatic habit that people either memorize or get used to. The Japanese equivalence would be,　In the set phrase, 気にしません...どうして”に”をつかえるの？誰も答えるできない.

OK. Thanks :)

It is how Englishman formed the word, just like other phrases right :)) ? I havent searched for thiss, but this is what i think : Impression is something catching your attention right? but it is just on the surface. you are impressed, but you dont't understand this from the inside out, i think this is why they use on here