Here's why:
* "-ful" forms adjectives: The suffix "-ful" is typically used to form adjectives. It means "full of" or "characterized by". For example:
* Hopeful (full of hope)
* Beautiful (characterized by beauty)
* Thankful (full of thanks)
* Adverbs modify verbs, adjectives, or other adverbs: Adverbs describe how, when, where, or to what extent an action is performed. They answer questions like "how?" "when?" "where?" "to what extent?"
Examples of adverbs:
* quickly
* happily
* loudly
* very
To make an adverb from an adjective, you often add the suffix "-ly":
* Hopeful (adjective) becomes hopefully (adverb)
* Beautiful (adjective) becomes beautifully (adverb)
* Thankful (adjective) becomes thankfully (adverb)
However, there are some exceptions where an adjective can function as an adverb, but this is usually in a less formal context.