✔ 最佳答案
You cannot omit 'in' in this sentence!
Since grammatical rules stated the to-infinitive in this case should delete the object if it that object has appeared before.
That means:He has a house to live in (the house). (the house:omitted!!!)
But if you have deleted "in",the verb 'live' will become a transitive verb rather than the intransitive verb and a grammar mistake will appear in your sentence!
Hope I can help you :)
2012-07-26 16:15:59 補充:
Sorry, the correct one is "if that object has appeared before." rather than " if it that object has appeared before.":)