We add 'ne' when it is a 'transitive verb'... transitive verb is a verb in which you can add the word "something" after it... for example "to eat something", "to drink something"...
But you can't say "to sleep something" or "to go something"... because they are 'intransitive verbs'...
Just add 'ne' if a verb is in simple past
Submitted report for "hover on" missing. I found this info about"ने": (http://www.learning-hindi.com/post/2392152114/lesson-83-the-past-tense-for-transitive-verbs)
ने is the indicator of who is doing (or in this case not doing) the action.
The action here is eating which was not done by Raj.
Hence the use of ने
So, the noun following ने is the doer. Or as we were taught in hindi "कर्ता ने" (Read about कारक http://hindi.swiftutors.com/hindi-kaarak.html)