PMI proves P(n) for all n >= n0 in two steps: (1) Base Case: show P(n0) is true. (2) Inductive Step: assume P(k) for arbitrary k >= n0, prove P(k+1). Both steps are essential — the base case anchors the proof, the inductive step propagates it. Common JEE error: assuming the result and "proving" it circularly. The inductive hypothesis must be explicitly used in proving P(k+1). Think of induction as a domino chain: base case topples the first domino, inductive step ensures each domino topples the next.