1) Ricardo's theory of rent states that rent arises due to differences in the fertility of land and that the least fertile land that is cultivated sets the price of farm output. 2) There are three situations where rent arises: extensive cultivation when more land is brought into use, intensive cultivation when more labor and capital is used on the same plots of land, and situation rent due to locational advantages. 3) Criticisms of the theory include that land fertility is not fixed, the concept of marginal land does not always exist, it ignores alternative uses of land, and factors like capital and labor are not homogeneous.