To give a counterexample, I have to find an integer n such n2 is divisible by 4, but n is not divisible by 4 — the “if” part must be true, but the “then” part must be false. Consider n = 6. Then n2 = 36 is divisible by 4, but n = 6 is not divisible by 4. Thus, n = 6 is a counterexample to the statement.