It is good to know that the words usually combined are adjectives. Adjectives are words that describe the quality/quantity/feature of a noun in a sentence. After adding "ly" such words become Adverbs. Adverbs are words that modify an adjective.
Adjectives \(+\) "ly" \(=\) Adverbs.
Some words from the lesson where "ly" is added to form a new word:
Words where "ly" cannot be added to make new words: