You can add relations to your tasks. To do this you first need to activate task relationships for your project. The activation can currently be done only via the web version the following way: Click on the info icon next to your project name on your project board, select manage and switch to the tab "Features" to activate "Task Relationships".
You can now add relations for tasks. Simply click on "Add..." next to the relations field on the right side bar of your task dialog.
A dialog will pop up allowing you to select the relationship of this task to other tasks.
Simply select the desired relation to add the relation. The task is now marked and you will be notified if you delete or complete a connected task.