Tagged after a verb, it carries the action on from where it stands — keep doing it, let it run forward without stopping.
框 · Frame
[verb] 下去 (keep V-ing on)
觸 · Trigger
An action is already underway and you want it to carry on forward from here, not stop.
序 · The move
1Set the verb for the action already in motion.Is this an act that can keep running, not a single instantaneous one?
2Tag 下去 to carry it on ahead from where it stands.Do I mean it continues forward (下去), not that it settles to a halt (下來)?
例 · Examples
1時間到了the time has come — it is up，我不能再cannot any longer說下去go on talking — the talking carried forward, kept running on了。
Time's up; I can't go on talking any further.
界 · Boundary
V下來 (aspectual: 停下來, settle to a halt)
下去 carries the action on away, forward with no stop; 下來 brings it to rest here, settling it to a halt. 去 walks on off ahead; 來 lands it back at the present. Continue vs stop, on the same 下 axis.
V下去1 (literal downward motion: 走下去, 跳下去)
There something physically descends, heading down and away. Here nothing moves down in space; 下去 is the figure of an action carried on through time. Literal path vs continuation.
V著 (durative: state held open)
著 holds a state open, frozen as it stands (門開著, the door stays open). 下去 pushes the action onward, running forward from now. Held still vs carried on.
我不能再說下來了。 → 我不能再說下去了。 (the talking is being carried on forward, so 下去; 下來 would settle it to a stop)
天氣這樣熱下來。 → 天氣這樣熱下去。 (the heat keeps running on into the coming days, so 下去)
請說下。 → 請說下去。 (下 alone leaves it unmarked; 下去 carries the talking on)
English says "keep talking," "go on reading" with a separate verb in front. Chinese tags 下去 behind the verb instead, and the continuation rides on 去 (carry on away), not on a word like "keep."