Most adverbs end in ly, but some don’t. Adverbs are a part of speech that are known for answering certain questions. These questions include how, why, to what extent, how often and where. There are a few others too. By definition adverbs are added to a sentence for descriptive purposes. They modify or describe verbs, adjectives or other adverbs.
However, they are best known for describing verbs. In the sentence, people might mistake the phrase “to eat” as a prepositional phrase because the word “to” is a preposition, but it is also used in part of an infinitive phrase. This is an infinitive phrase in this sentence and not a preposition. The word inside describes where the boys went, so it is an adverb.