Hi,

My wife (Finnish) and I (English) are trying to work out when 'Mukaan' should be used for 'with' and when 'Kanssa'

It's the age old story that she knows instinctively when to use the right one on her own language but doesn't know the rule to explain to a foreigner

If anyone can explain it would be just ihana.

kanssa - with two equal comparable things. postposition + gen.

example
Isä poikansa kanssa
Riitelimme Pekan kanssa
Ohukaisia hiilon kanssa

mukaan - with, together. postiposition and also adverbium.

example
Lähde mukaan
Tule minun mukaani
Painon mukaan määrätty hinta
Suola maun mukaan

Hi, just wanted to say thanks, although belatedly. I didn't get notification of the reply and I forgot I'd posted this. It was nice to come one here and find this waiting for me