Most adjectives are followed by to infinitive:
pleased, glad, surprised, disappointed, relieved, shocked, interesting, kind, hard, essential, difficult, easy
I was pleased to receive your invitation.
I was glad to meet my old friends again.
We were surprised to get a bill for £45.
We were sorry to hear your bad news.
It was kind of you to invite me.
It was easier to do than I had expected.
He was afraid to open the door.
The exercise was hard to do.
BUT: many common adjectives are followed by a preposition + -ing
afraid of, interested in, sorry about, good at, capable of, famous for, fond of
She´s afraid of flying.
We´re sorry about interrupting.
He´s fond of collecting butterflies.
They´re very good at skiing.