There are several ways to view completed tasks:

Search

You can check the box to include completed tasks in a basic search, or you can specify a % complete in the advanced search form.

Hide/Show Completed Tasks Control

If you are looking at the tree view of a task (you’ve clicked on the task title to make it the root of the tree view), you can set the Hide/Show Completed Tasks drop down at the bottom of the page to show completed tasks.

Note: the Hide/Show Completed Tasks drop down is not available on the Home screen.