In the sentence "The amount of feedback that students receive is not optimal", the word "optimal" is an adjective, more specifically a predicate adjective modifying the simple subject of the sentence, "amount". The word "optimum" does not occur in the sentence, although "optimal" could reasonably be called an adjective corresponding to the noun "optimum". In informal speech, "optimum" itself is often used as an adjective, but "optimal" is preferable for the adjective form.
optimum combination of input
Option, Optimal, Optimum, optional?
The optimum pH for sucrase is 2.0
optimum pH is 7.3
There was a maximum tax increase in 2009, but our economy is trying to patch up.
· opportunity
the optimal temperature of the small intestines is 37.
For optimum results follow directions.
choice, best, select, perfect, optimum, ace, excellent, solid
the guy on the hockey team was the optimum player
It was the most optimum outcome possible, given the complicated circumstances. It was the optimum time to ask her about hosting a party.
The optimum way to make your parents happy is by getting good grades.