How do I soft delete a thing?

Use the "alterThing" action to set a thing's "status" to "inactive".

How do I undelete a thing? 

To undelete a soft-deleted thing, use the "alterThing" action to set a thing's "status" to "active". 
You cannot undelete a thing that has been deleted using the "deleteThings" action.

How do I list only active things?

Use the "listThings" action with "status" set to "active".

How do I list soft-deleted things?

Use the "listThings" action with "status" set to "inactive".

How do I clone a thing?

Use the "describeThings" action to retrieve a thing and use its properties in "createThing" to create a new thing. You will need to give the new thing a different name.