How to do something when we press Add New Row in child table

Thank you a lot for the kindly help.
onload will be trigger when loading the document.
refresh will be trigger when doing refresh for the page.
changed_field_in_child_table: will be trigger only if the specified field (mentioned) is changed, and not any field. In other words, we have to specify the field that need to be changed to be trigger the event.

Still I am looking to know an even that will be trigger when any field (without specifying the field) will be changed.

Appreciate the kindly help.

I assume there are only a few columns in your child table.
In that case My suggestion is that write a common function, trigger all fields and call the funtion in all triggers

OK, so there is not a method to be triggered once any field is selected.
From the other side:
If the trigger happened at row #3 and I need to change the value at row # 2, how I can do it? I know that using locals[cdt][cdn] will let me do changes on the current row, but how I can do changes on the previous row of the changed row?

Please try looping through the child table rows as an array of objects - frm.doc.child_table and do your stuff.