For forming numbers with constraints: (1) identify position constraints (first digit nonzero), (2) handle divisibility (last digit for div by 5, last two for div by 4), (3) count using multiplication principle position by position. Always start filling the most constrained position first. Example: 4-digit even numbers from {0,1,2,3,4} without repetition — fill units place first (0,2,4 = 3 choices), then handle thousands (nonzero), then remaining.