«دخل زيد فعمرو»
Zayd entered and Amr followed in close succession.
«دخل زيد ثم عمرو»
Zayd entered, and then Amr entered after an interval.
In both cases Amr comes second. Faa, however, expresses close succession, whereas thumma expresses delay. The delay is not a fixed number of clock minutes; its significance depends on the action and context.
Overlooking the difference can distort the time relationship in a vow or conditional legal declaration.