This is the first entry in a planned series aiming to establish a modified, and simpler, formalism for studying the geometry of smooth manifolds with a metric, while remaining close to standard textbook treatments in terms of notation and concepts. The key step is extending the tangent space at each point from a vector space to a geometric algebra, which is a linear space incorporating vectors with dot and wedge multiplication, and extending the affine connection to a directional derivative acting naturally on fields of multivectors (elements of the geometric algebra). A short introduction to geometric algebra is included in the text. The theory that results from this extension is simpler and more powerful than either differential forms or tensor methods, in a number of ways. The multivector directional derivative obeys a powerful product rule. Simple conditions are obtained for metric-compatibility and torsion-freeness of the connection coefficients, and derivatives with torsion are treated generally. The covariant derivative of tensor fields is derived from a simple chain rule. Arbitrary metric signatures are treated in generality. The curved-manifold equivalents of the gradient, divergence, and curl operators are investigated, and the torsion-free curl is shown to be equivalent to the exterior derivative of differential forms. Unlike most traditional treatments, the entire formalism is developed in terms of completely arbitrary vector bases, which might be neither coordinate bases nor orthonormal. Methods of geometric algebra have previously been applied to vector calculus with great success, and extended to curved spaces in several ways (notably including "vector manifolds" and "gauge theory gravity"). Here we provide a new way of extending geometric calculus to curved manifolds, which more closely parallels standard Riemannian geometry.