quenya_grammar1-8  Exercise 1: Aorist and Present
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              car- sg.   car- pl.   henta- sg.   henta- pl.
  Aorist      carë       carir      henta        hentar
  Present     cára       cárar      hentëa       hentëar

Plural ending: -r (carë -> carir; henta -> hentar).

Aorist vs present:
  car-    the present lengthens the vowel (carë -> cára)
  henta-  the present inserts -ë- (henta -> hentëa)
The page gives no general rule; these are patterns from two verbs.