As it was told To Do module could use hierarchical structure. To create sub-To Do select parent To Do and press Add as Child button

You can use Move In/Out and Move Up/Down buttons to edit hierarchical structure. It's also possible to drug & drop To Do's to each other to edit hierarchy.

Collapse/Expand All buttons easily collapse/expand the whole tree structure:

The module has two main views: Table and Tree. You can switch between them using buttons on the left pane:

Table view ignores all the relationships between To Do's and shows them all as root entries.

Tree view shows hierarchical structure of To Do's.

Hide Completed checkbox works on all completed entries. Completed root entry, will not, however, be hided, unless all sub-entries are completed.