Błędnie: Sherlock knocked to the door: it sounded as if there was someone behind it. Poprawnie: Sherlock knocked at/on the door: it sounded as if there was someone behind it.
Słowo ‘knock’ znaczy „pukać” i łączy się z przyimkiem ‘at’ lub ‘on’, np. ‘She was knocking at/on my door’ („Pukała do moich drzwi”).