tap on：轻轻敲击(某表面)，轻击…(某部位) beat surface of sth，hit sb/sth lightly on (a part)。
on tap：随时供应的,现成的。这里tap是名词，ready for use when needed。
【例句】
Someone is tapping on the door.
有人在敲门。
The teacher tapped her fingers on the desk impatiently.
老师不耐烦地用手指敲桌子。
【例句】
Someone is tapping on the door.
有人在敲门。
The teacher tapped her fingers on the desk impatiently.
老师不耐烦地用手指敲桌子。
