Difference between tard and en retard

Question

Bonjour!

Mon frère est arrivé ________ chez le dentiste ce matin !My brother arrived late at the dentist's this morning!In the quiz, the correct answer was "en retard", while "tard" was marked as wrong.

May I know why "tard" cannot be used in this sentence?

Céline Pickard · Accepted Answer

Bonjour JC,
In this context (i.e. having an appointment, which involves making sure you arrive at a very specific time), you can only have "en retard". 
Mon frère est arrivé en retard chez le dentiste = My brither arrived late at the dentist's
Mon frère est arrivé tard = My brother arrived late
-> "tard" "expresses the quality of lateness usually with respect to the time of the day or some other event" (see the following link: tard et retard)
I hope this is helpful.
Bonne journée !