He knocked once on the door, then opened it and stepped inside.

He knocked once on the door, and stepped into the main building.

She knocked once, then again, and finally the head of a very sleepy Japanese man appeared.

She knocked once at the door and walked straight in.

She knocked once, then opened the office door and looked inside.

He approached the rear door and knocked once on the window.

He knocked once on the screen door of the kitchen and went straight in.

When the general believed himself to be alone, he knocked once; but all was silent.

The question is, why the wall, once knocked down, was reconstructed.

Taking a deep breath, she knocked once and stepped inside.