Direct substitution evaluates limit by plugging x=a into f(x) if function continuous at a. Works when: (1) f(a) is defined; (2) Result is not indeterminate form. Steps: (1) Substitute x=a into f(x); (2) Simplify; (3) Result is limit. Example: lim(x→3) (2x + 5) = 2(3) + 5 = 11. Fails for: (1) 0/0 form; (2) ∞/∞ form; (3) 0×∞; (4) ∞ - ∞. In these cases, use factorization, rationalization, L'Hôpital's rule. Exam tip: Always try direct substitution first. If indeterminate, algebraically simplify. Understand when direct substitution works vs fails.