Words ending in the suffix - ly are adverbs. Words not ending in -ly may serve as an adjective or adverb, depending on the context of the sentence. In the sentence you gave: I enjoyed sitting close to the windows and looking at the view I (subject noun) enjoyed (past tense verb) sitting (gerund [verb acting as a noun]) close (adjective, since it is describing a gerund) to the windows (prepositional phrase) and looking (gerund [verb acting as a noun]) at the view (prepositional phrase). Close to the school, there was a beautiful park with many trees around it。 Close (adjective, since it is describing a prepositional phrase, which acts as a noun) to the school (prepositional phrase), there was a beautiful (adjective, since it described a noun) park (subject noun) with many trees (prepositional phrase) around it (prepositional phrase).

In this case: closely - adverb of manner - example: "The jeweler looked closely at the diamond ring." close - adverb of position - example: "The jeweler was sitting close to the door." close - adjective of position - example: The jeweler sat down at the table close to the door." "I enjoyed sitting close to the windows and looking at the view." Close is an adverb of position that modifies the gerund sitting. More information about adverbs modifying gerunds. https://simple.wikipedia.org/wiki/Gerund The gerund can be modified by an adverb: Breathing deeply helps you to calm down. "Close to the school, there was a beautiful park with many trees around it." (= "There was a beautiful park close to the school. There were many trees around the park.") Close is an adjective of position that modifies park. More information on close as an adjective and an adverb - New Oxford American Dictionary close | kloʊs | adjective 1 a short distance away or apart in space or time: the hotel is close to the sea (my note - The adjective close modifies the noun hotel.) adverb in a position so as to be very near to someone or something; with very little space between: they stood close to the door (my note - The adverb close modifies the verb stood.)