knock : 指用拳头或其它器械猛敲或猛打；也指用手轻敲某物。
- She knocked on the window to attract his attention.
- There's someone knocking on/at the door.
- Please knock before entering.
tap : 多指慢慢地连续地轻击或轻拍。
- The branches tapped against the window.
- I could hear him tapping his fingers on the desk.
- I was tapping my feet (= hitting the floor gently with my feet) to the music.
- Someone tapped me on the shoulder.
rap : 指用手指或木棒等快速地轻敲或急拍。
- She rapped (on) the table to get everyone's attention.
- The colonel rapped (out) an order to his men.
pat : 指用手轻拍以示同情、赞同或爱抚。
- He patted my head/patted me on the head affectionately.
- I bent down to pat the little puppy.