The phrase "from then on" means starting at a certain point in time and continuing thereafter. Some synonym phrases that can be used in its place include "thereafter," "since that moment," "from that day forward," "starting thereon," and "following that." Each of these phrases conveys the same meaning as "from then on" and can be used interchangeably. The choice of a synonym phrase may depend on the context or writing style. Regardless of the phrase used, it is important to convey the sense of a clear starting point that sets the timeline for events that occur thereafter.