"DuplicatePreviousInput" is a front end token that duplicates the contents of the nearest input cell above.

"DuplicatePreviousOutput" is a front end token that duplicates the contents of the nearest output cell above.

Invisible characters. In the input there is an invisible comma between the 1 and 2. Here there is an invisible space between the x and y, interpreted as multiplication.

Getting parts of lists. This gives a list of parts 1 and 3. Here is a nested list.

"DeleteNext" is a front end token that deletes the character to the right of the insertion point.

"DeletePrevious" is a front end token that deletes the character to the left of the insertion point.

AutoDelete is an option for boxes that specifies whether a box is automatically deleted when its contents are edited.

Drop[list, n] gives list with its first n elements dropped. Drop[list, -n] gives list with its last n elements dropped. Drop[list, {n}] gives list with its n\[Null]^th ...

Mathematica provides a large number of options for cells. All of these options can be accessed through the Option Inspector menu item in the front end. They can be set either ...

Mathematica encapsulates in a small number of functions vast flexibility in rearranging lists with any structure and any number of elements.