The simple answer is that you probably can't. I'll try to explain:
Unless you're very young, you probably used to use a VHS video cassette recorder to record TV programmes while you were out. Alternatively, you could record one programme, while watching another. This was because the set-up you were using contained two TV tuners. There was one inside the TV and another one inside the VCR. This meant that you could use the tuner inside the TV to feed the programme you watched 'live' to the TV screen while the tuner inside the VCR could be used to record another channel. When you went out it didn't matter that your TV was turned off because the tuner inside the VCR was still switched on and able to provide a signal for recording on tape.
DVD recorders vary but many cheaper models (almost certainly including your Liteon machine) don't have a TV tuner built into them. Instead, they rely on the tuner inside the TV to provide the signal which is recorded onto the DVD. This means that there are two important differences to the way that a VCR works. Firstly, you can't record one programme while watching another (because that requires two tuners and you've only got one). Secondly, you can't record a TV programme when the TV is switched off (because that means that the only available TV tuner is switched off).
The remedy is to upgrade to a DVD recorder with a built-in tuner or to introduce another TV tuner into the set-up (e.g by using a Freeview box and recording the signal from that).
Chris