The Bronsted-Lowry Theory states that weak acids have strong conjugate bases and weak bases have strong conjugate acids. The weaker the acid or base, the stronger its conjugate partner. A weak acid is weak because its conjugate base is strong enough to reform the original acid, and similarly for weak bases and their conjugate acids. For example, NH3 is a weak base because its conjugate acid NH4+ is strong and readily gives up H+ to reform NH3.