جَاءَنِي زَيْدٌ فَعَمْرٌو
Zayd came, immediately followed by ʿAmr.
جَاءَنِي زَيْدٌ ثُمَّ عَمْرٌو
Zayd came and, after an interval, ʿAmr came.
Both conjunctions express ordering, but «fa» suggests close succession while «thumma» marks relative delay. Translating both merely as an unqualified “then” risks losing the distinction.
The relevant choice follows the speaker's intention: an immediate sequence requires different wording from a sequence with a notable interval.