Can you tell me some of system tasks and their purpose?

$display, $displayb, $displayh, $displayo, $write, $writeb, $writeh, $writeo.

The most useful of these is $display. This can be used for displaying strings, expression or values of variables.

Here are some illustrations of usage.

$display("Hello oni");

--- output: Hello oni

$display($time) // current simulation time.

--- output: 460

counter = 4'b10;

$display(" The count is %b", counter);

--- output: The count is 0010

$reset resets the simulation back to time 0; $stop halts the simulator and puts it in interactive mode where the user can enter commands

$finish exits the simulator back to the operating system.