When there is an "on" after touch?
Is it correct that I say"I touch on the book" or just "I touch the book" Sometimes I come across "touch on" in some abstract meaning, what is the difference?
touch on (something) TFD is an idiom
To discuss or deal with some topic informally or in passing.
As in:
We'll touch on that matter later in the meeting.
to touch v. TFD A basic definition is
To cause or permit a part of the body, especially the hand or fingers, to come in contact with so as to feel