Complete Sentences

"Robert did" is a complete sentence, but only because it's an answer to a question. Otherwise, you'd need to fill out the complement. It's also possible to say "Robert ate" in a different context. In the answer to this question, though, the verb becomes transitive. It needs an object.

Here's a quick thing about transitive vs intransitive verbs:
http://www.quickanddirtytips.com/education/grammar/transitive-and-intransitive-verbs

One-word complete sentences are also possible, if it's a command and the subject is implied by context. "Eat!"

Thank you so much!

No sweat. Hope it helps.