when do I need use: "input." and when I shouldn't

when do I need use: "input." and when I shouldn't

Hi:

Can you let me know when I need use: "input." and when I shouldn't use that?

For example:
input.Today_Date = zoho.currentdate; //this is correct. (get today's date and show on the Today_Date Field)
disable input.Today_Date;                  //this syntax is not correct.
disable Today_Date;                          //this is correct. (set this field read only).

Thanks