Why Lesson Planet?
Here is a useful summary of the definitions and rules governing use of positive, comparative, and superlative adverbs. Nice distinctions made among -er and -est endings and adverbs that use more and most. It's so thorough, it makes explicit that one should not use both forms at the same time (e.g. more faster). At the end, learners categorize six adverbs as positive, comparative, or superlative.