Meaning: " From a particular day / event ( still to come ), they'll be(come) friends for life. " I've read here that the idiom " from then on " is only used to describe an event already happened, from the past. If so, what can I say to replace " from then on " in the sense of my example ? " These two boys do not stop arguing. In my opinion, they would need a good little fight. From then on ( Google suggests from there ), they will be the best friends in the world. An idiom / expression in 3 syllables needed, just like "from then on".