You can attach an existing task as a subtask of another task in just a few clicks.

First, choose the task you want to attach, it can be a task or a subtask. Second open the task and click on the three dots and select Choose this task from the dropdown menu.

Third open the task you want to attach the first task to, as a subtask, it can be a task or a subtask. Click on the three dots on the top right of the task card and select Link chosen task.

Fourth, a confirmation prompt will appear to make sure the operation is the one you expected. If so, confirm and the first task will now be a subtask of the second task. If the first task already had a parent task, the parent is replaced by the second task.

Here is a quick GIF:

Here are a few cases where this operation cannot be completed: