You are saying more about someone already named, and the same one is still acting.
序 · The move
1Name the doer once, in the first clause.Is the referent now active for the listener?
2In the next clause, leave the subject slot empty and start straight at the verb.Is the same one still acting? If the actor switches, name the new one.
3Carry the empty slot across as many clauses as the doer holds.Has anything changed who is acting? Re-name the moment it does.
例 · Examples
1他看to look at了菜單menu後after，決定to decide點to order (food)一份one (portion)三明治sandwich。
He looked at the menu, then decided to order a sandwich.
界 · Boundary
overt pronoun 他/她/它
The pronoun is set down when the doer is new, switched, or stressed; the slot is left empty when the same doer simply continues. Re-name on a change, drop on continuation.
Topic-comment fronts the thing under discussion and remarks on it; the zero pronoun is what lets the following clauses run on with no subject at all. The topic feeds the empty slot.
他看了菜單後，他決定點一份三明治。 → 他看了菜單後，決定點一份三明治。 (same doer continuing — leave the second slot empty)
他看了菜單後，決定點。 → 他看了菜單後，決定點一份三明治。 (the subject drops, the object cannot — name what is ordered)
他點了菜，很快端上來。 → 他點了菜，服務生很快端上來。 (the actor switches to 服務生 — name the new one, do not drop it)
English re-states he/she/it in every clause; learners carry that over and repeat the pronoun where Mandarin leaves the slot empty.