Basic: Current flow and grounding
Keep in mind that an "ideal ammeter" is the equivalent of a wire for understanding what the circuit does, and that current with only flow if you have a loop.

Also, note that the built-in schematic tool will let you draw and simulate these circuits quickly and easily to answer questions like this!  You can start with my examples below.

Final Note: A "1.5 ohm LED" is not a real thing.  I have substituted 1.5 ohm lamps in the schematics below.  I won't go into details on the LED/lamp issue here because the lamp is just a hypothetical component filling the role of whatever load is connected to this circuit.  I'm sure a little research will clarify how LEDs work.

Case 1: Yes, the ammeter reads 1A.  







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Case 2: Here you have disconnected the negative terminal of the battery.  There's no loop through which current can flow, and the ammeter reads 0A.







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Case 3: This case is EXACTLY the same as case 1, because for all practical purposes, an ideal ammeter acts the same as a wire.  Because ammeter 1 and ammeter 2 are part of the same current loop, they must read the same value - although here, ammeter 2 reads -1A because the current flows into its negative terminal instead of its positive terminal (clockwise around the loop). 







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So, what about that ground fault interrupter?

The key concept here is that the neutral wire and the Ground wire are both ~0V.  However, the neutral wire is INTENDED to carry the return current.  The ground wire is NOT intended to carry any current.  

The circuit at left is the "intended" operation.  Notice that the two ammeters on the Hot and Neutral wires read the same 1A.

In the circuit at right, a "person" (shown as a 100mOhm resistor) makes a connection from the Hot wire to the Ground wire (or, the grounded metal case of the circuit). Current is now flowing along the Ground wire, and the Hot and Neutral wires now have different currents.  This triggers the GFI circuit and turns off power to the lamp. 







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