loud, sharp
gentle, light, quiet, soft
timid
sudden

hear
answer
She hurried to answer the knock at the door.

a knock at the door, a knock on the door
There was a loud knock at the door.

hard, nasty (esp. BrE)
the hard knocks of life (figurative)

minor (esp. BrE)

get, have, take
You've had a nasty knock on the head.
Their pride took quite a knock when they lost 5–0. (figurative)

give sb/sth

knock on

accidentally
I accidentally knocked the vase over.

against
The stick knocked against the wall.

off
He had knocked one of the pictures off the wall.

on
I knocked my head on one of the beams.

knock sb/sth flying
He was knocked flying as two policemen came crashing through the door.

knock sb off balance, knock sb off their feet
The explosion knocked him off his feet.

knock sb senseless, knock sb unconscious
The blow knocked him unconscious.

knock sb to the ground

loudly
gently, lightly, quietly, softly
politely, timidly

at
Someone knocked loudly at the door.

on
She knocked timidly on the study door and entered.

without knocking
Dobson walked straight into her office without knocking.

Knock is used with these nouns as the subject:
blow, knee

Knock is used with these nouns as the object:
nail, shin