1. I am sorry that I didn't meet you before you left.
2. I know the girl he has married/married. -- both OK
3. He broke the plate he ate lunch on.
4. I saw the house he has bought/bought. -- both OK
5. He has not /did not come. -- 'has not': they are still waiting for him. 'Did not': they have given up waiting or it is too late.