The comparative ending in –er is not used to compare two qualities of the same person or thing. For example, if we want to say that the smartness of John is greater than his prudence, we use a structure with more.

• John is more smart than prudent. (NOT John is smarter than prudent.)

To compare the same quality present in two people or things, we use the regular comparative forms ending in –er.

• John is smarter than Peter. (= The smartness of John is greater than that of Peter.)

Related posts:

1. Degrees of Comparison
2. Formation of the Comparative and the Superlative