Comparative of Disyllabic Adjectives

In English monosyllabic adjectives usually attach the suffix -er to express the comparative and adjectives with three of more syllables use the word more. The suffix is known as the synthetic form and the adverb as the analytic. However, in disyllabic adjectives there is often variation.

Disyllabic adjectives with word final -y (optionally with -ly) form the comparative with -er. This is the case with the words happy, lovely and pretty. Other disyllabic adjectives form the comparative by inserting the word more. This is the case with pleasant, solid and vivid.

Certain disyllabic adjectives can be formed with the synthetic or the analytic form. The following words show variation:

clever 
common
friendly
gentle
likely
narrow
polite
quiet
simple
subtle

Here is how I express the comparative of these adjectives:

Synthetic A + -er

friendly
likely
narrow
quiet
simple

Analytic more + A

clever
common
gentle
polite
subtle

The comparative of disyllabic adjectives often varies. The form that speakers choose for the comparative tends to be the same for the superlative. It may be that the analytic form has become more common than it was in the past.