There's an example:
He (kill) his victim easily, (remove) his wallet, (wipe) away all fingerprints and just (make) for the front door to make his escape when he (hear) a key turning in the lock. Someone (come) in! What he (do) if he (be) seen?
And I know that it should be: killed, removed, wiped, made, heard, was coming. But the answer in the last sentence is 'would do' and 'were seen'. I don't understand why I can't use 3rd conditional - 'would have done', 'had been seen'.