once you save&submit the task is closed and you can not change it anymore.

in order data are saved to list item with save, you need to place list item field reference on the form resp. connect a form control to list item field.

if you are not able to get it working you should provide more info on what you exactly doing/trying and what you want to accomplish.

The problem is that the task forms save the information to a "Workflow tasks" list, not the original list. Then it seems I need a second workflow in the WF tasks list in order to copy the entered info to the original list... that's cumbersome?

when you look on left-side pane of task form designer you should see 3 column categories there. list columns, task columns and content type columns.

you you want to save data from task task from directly to list item you have to choose from list columns.

if you are talking about additional data being queried with customized content type, then it's true they are saved just to task list by default, but in action configuration dialog you can bind content type columns to workflow variables.

then you need to update list item with data collected in workflow variables on your own. so no need for an extra workflow either.