I have a philips hard disk dvd recorder, and I imagine they all work the same way.
I can't recordings that are already on the hard drive whilst recording, or watch a recording from the beginning, even if it is still recording the programme.
I can pause live tv.
I can also watch a different channel whilst recording BUT not through the DVD recorder. I simply change the channel on the tv, exactly as you would with an old video recorder.
If my dvd recorder had a twin tuner, I could record one channel and watch another through the dvd.