Philip“The sentence is too difficult to understand.
The student is too ignorant to understand.”
The specific adjective determines whether the subject of the sentence is the subject or the object of the infinitive.
difficult goes in the same group as easy, tough, hard, ...
ignorant goes in the same group as eager, pleased, ...
Once in a while you'll run across an adjective that can go either way, giving an ambiguous sentence:
The chicken is ready to eat.
CJ