All adverbs ending in -ly and certain other adverbs, especially long ones, form the comparative by the addition of the word more or less. Some short or irregular adverbs, however, form the comparative by the addition of the ending -er.
The experiment went better than expected.
[well changed to comparative form better with ending -er]
We got our results more quickly than we expected.
[comparative with more]
The solution reached equilibrium less rapidly than we thought
it would. [comparative with less]
