Comparison (using Ellipsis)

I am trying to figure out when do we need to use an action verb explicitly and when can we omit it using the (ellipsis concept). For Example:

• John is taller than Jim [is] (I understood that here is can be omitted)
• John left earlier than Jim [did] – is it ok to omit the word ‘did’ – if not, why?
• Maple trees shed their autumn leaves earlier than oak trees – Is this correct?

Thanks for your input