OK...for the question about guitar, one lead in 5 and one lead in 6. Not sure why you would do that. Best to put one in 3 and one in 4 (or any two of the first four inputs) so you can set your levels separately and pan them accordingly.
Line level signals come from things like keyboards and drum machines, and usually, since these things are stereo (2-channel) signals, it's best to input them in 5-6 or 7-8, bypassing (as James points out) extraneous things like the trim control and EQ. And you can set the levels of the stereo signal with only one fader. By the way, this doesn't mean you HAVE to record a stereo signal; if you record only to one track, the signal will be mono. BUt if you leave your stuff plugged in there (like I do) it's nice to have that option to go stereo.
Also, as James points out, an outboard preamp, or direct box can be used in lieu of the preamps in the portastudio, so inputting a guitar direct this way bypasses much of the portastudio's signal chain (the trim and EQ), thus giving you a signal with less noise.
The general point of those extra inputs is to bring in line level, stereo signals from things like keyboards, drum machines, or effects returns, though they can be used in other ways. Whereas the first four inputs are more for single channel, mic level (mic'd) signals. So while technically if you had a keyboard and wanted to record a stereo signal you could plug the stereo outputs of the keyboard into, say, channels 3 and 4, it's better to use the next series of inputs (5-6, 7-8) for such things, leaving 1-4 open for mic inputs. Hope this helps!
[This message has been edited by MM2 (edited 06-28-2000).]
[This message has been edited by MM2 (edited 06-28-2000).]