It seems you are doing dangerous things at home. What a knife (is that the only tool you have?) is doing close to wire without shutting off power (I mean be sure at 100% that there is no active phase).
I strongly advise you to stop this kind of activities unless you exactly know what you are doing, use proper tools and prepare your work as professional does.
Anyway tempting to answer your questions:
1) Is your light switch bipolar? Does it break two wires or just one? If it is unipolar, there is still an active wire when you are working. You must be aware of that before working on the circuit. You are not supposed to discover it after.
2) What do you mean by neutral? A blue wire? What is your earthing system? If you are in a domestic installation, you likely have a TT schema and therefore you have no neutral distribution but two active phases.
3) A fault-current breaker will switch off when 30mA or 300mA (depends of its characteristics) flows from any phase to earth. If there is no load, it is just limited by the wire impedance, make your own conclusion. In your case, I am pretty sure you simply shortcut a phase to ground and the circuit breaker saved your life (I am not joking, domestic injuries - including death - with electricity happens the way you did, not like in your last post).
4) It is a common mistake to believe that neutral is not an active wire. For a three phases load no current will flow through neutral if all phases are balanced (this never occurs in reality), this case is a theoretical exception. Other way, such in TN earthing system, neutral is collecting a huge amount of current. Neutral IS an ACTIVE wire.
When dealing with electricity you must at least use protection (there are circuit breaker, you haven't used them), correct tools (where is your multimeter to check that the circuit was broken?) and plan your actions before performing the maintenance.