When does the output of a NOR gate =0?

A NOR gate is built from an OR gate followed by a NOT (inverter) gate, so its output is always the opposite of what a plain OR gate would produce for the same inputs.

An OR gate outputs 1 as soon as at least one of its inputs is 1, and it outputs 0 only when every input is 0. Because the NOR gate inverts this result, the NOR gate does the reverse: its output is 1 only when every input is 0, and its output becomes 0 as soon as at least one input is 1. This can be shown with the truth table for a two-input NOR gate:

The table shows that the output falls to 0 in every row where at least one input equals 1, which matches the description of the output falling to 0 whenever 1 is present at input.

Exam takeaway: a NOR gate is the exact logical opposite of an OR gate, so wherever OR would output 1 (any input is 1), NOR outputs 0, and wherever OR would output 0 (all inputs are 0), NOR outputs 1.