Modifiers—adjectives and adverbs—attribute qualities to nouns and verbs. When the modifier shows a greater quality, it appears in the comparative state.
Usually, for adjectives, you form the comparative state by adding “-er” to the adjective, as in the hotter plate. But you would not describe a test as difficulter. You would write a more difficult test.
Various rules govern the use of “-er” or more. You'll find a complete discussion in the sections on Adjectives and Adverbs in Parts of Speech on Grammar.com. Click here for the discussion on adjectives and here for adverbs.
See positive state and superlative state.