Circonflexe on plaît

Question

I noticed that only the 3rd person singular has the circonflexe on the i (Tu plais à Luc vs Elle plaît à Luc ).  Is there a reason for this? Just curious.

Accepted Answer

There are several reasons, some words take the circumflex. For instance, a letter that has „disappeared“ (forêt was once forest, hôpital was once hospital, etc.). Another reason is to distinguish them from their brethren without circumflex (pâle=pale, and pale=plank, rudder). But I couldn‘t find a reason why plaire takes on a circumflex in the third person singular: plaît. 
Anyway, here is a pretty comprehensive summary: https://www.francaisfacile.com/exercices/exercice-francais-2/exercice-francais-56617.php

Answer

In Old French, the 3rd person singular was plaist, so it's a missing "s" as Chris suggested.https://www.verbix.com/webverbix/go.php?T1=plaire&Submit=Go&D1=12885846&H1=12885846