I'm having trouble getting my head round input connections; I'm trying to attach an object to a motion curve but Maya says no can do because the object already has an input connection. Poking around in Hypergraph InputOutput hasn't made me any wiser. Where in Help can I read up on this? or better still what's the fix?
Channels / attributes are just places to plug in information--usually numbers. In general you can't fit two numbers into one spot. Consider transformation: if one input was telling a cube to be at x = 10 and another input was simultaneously trying to tell it to be at x = 20, how would that make sense? There are special nodes that can blend, average, or choose between multiple inputs, but in general, you've got to have just one piece of information going into a particular slot.
The Hypershade can sometimes be a little more user-friendly than the Hypergraph. Look for colour-coded connections: blue for a single attribute, cyan for double (like UV info), green for a triple attribute (like colour, or position).
In this case, it sounds like you're perhaps trying to apply path animation to an object that already has some keyframes on it. You can't cram both keyframe info and path info into the object's transform that way--how would it decide where to go?
Solutions would be to delete the keyframed animation, or to group the (animated) object and then attach the group to the path, not the object itself. This is common practice, to allow the animator to retain control over the object for fine-tuning motion without having to change the path animation settings.