I can't give you a detailed explanation without knowing what sentence you heard "för" in, as prepositions sometimes don't translate directly and need context, but I can tell you that "Ankan simmar för flickan" in this case would mean that the duck swims FOR the girl. Personally it gives me an image in my head of the duck showing off its swimming/putting on some kind of show to the girl.
No, but you could say that with på as a particle (stressed). You can use this with most words of movement: köra på 'drive into', 'hit' (with your car) cykla på 'drive into someone/thing while riding a bike', segla på 'hit someone/thing by sailing' etc etc.
'I ran into him' would be Jag sprang på honom – remember to stress 'på' to make it a particle, as a preposition the meaning will be very different.
Hi, if I understood correctly på is used to express a contact and also some kind of a blow, I mean some kind of violence in the contact; while till is used to express the complete movement from point A to B but without a contact, am I right? Or it could exist a contact but without any kind of violence in it? Thanks.
That sounds pretty correct, yes. "På" implies some kind of collision, intentional or not, while "till" implies non-violently stopping somewhere, either at a location or by something or someone. So yes, "till" can imply contact (at least if I understand what you mean by contact right), like for example in this very sentence, "Ankan simmar till flickan."