1. A classic example of Gothic architecture. The word "classic" here is used to emphasize that the example is typical or representative of Gothic architecture as a whole. When we use an adjective to describe a noun in a general sense, we usually use the indefinite article "a" before the adjective. In this case, "classic" is being used to describe the noun "example," so we need to use the indefinite article "a" before "classic." 2. To be characteristic of US imperialism. The phrase "characteristic of" is used to describe a quality or attribute that is typical of something. In this case, "characteristic of" is being used to describe the noun phrase "US imperialism," indicating that the example refers to a quality that is typical of US imperialism in general. Since "characteristic" is an adjective modifying the noun phrase "US imperialism," we do not need to use any articles before these words. 3. A book on ancient history. The noun "book" here is referring to a specific, but unnamed, book. When we refer to a specific but unnamed item or thing, we use the indefinite article "a" before the noun. In this case, "book" is being used to refer to a generic book about ancient history, so we use the indefinite article "a" before "book." 4. Devoted to Flemish painting of the 17th century. The phrase "devoted to" is used to describe something that is focused on or centered around a specific subject or topic. In this case, "devoted to" is being used to describe the noun phrase "Flemish painting of the 17th century," indicating that the example refers to something that is focused on or centered around Flemish painting of the 17th century. Since "devoted" is an adjective modifying the noun phrase, we do not need to use any articles before these words. 5. A book on the history of ancient poetry. The noun "book" here is referring to a specific, but unnamed, book. When we refer to a specific but unnamed item or thing, we use the indefinite article "a" before the noun. In this case, "book" is being used to refer to a generic book about the history of ancient poetry, so we use the indefinite article "a" before "book." Additionally, the phrase "the history of" is used to specify the subject or topic of the book, indicating that it is specifically focused on the history of ancient poetry. 6. Three o'clock by Greenwich time. The phrase "three o'clock" is a specific time. When we refer to a specific time in this way, we do not use any articles before the time. In this case, "three o'clock" is being used to refer to a specific time as determined by Greenwich time. We do not need to use any articles before "three o'clock" or "Greenwich time."