Some facts are easy enough to memorize.

to memorize is the infinitive
is it used as an adverb?

Yes. It's an adverb modifying the adjective, "easy."

The word "to" in the sentence "Some facts are easy enough to memorize" is not an adverb. In this case, "to" serves as part of the infinitive verb phrase "to memorize." Infinitives are commonly used in English to express purpose or intention. In this sentence, the phrase "to memorize" describes the purpose or goal of the action being described.

To identify adverbs in a sentence, you can look for words that modify verbs, adjectives, or other adverbs. Adverbs often end in -ly, but not always. They can also be formed from adjectives by adding -ly, such as "fast" becoming "fastly" to form the adverb. Additionally, "to" can also be used as a preposition in other sentence structures.