"Greatly" Natural Recordings by Native Speakers
The adverb "greatly" means to a large extent or degree; extremely or significantly. It is often used to emphasize the intensity or extent of a feeling, action, or situation. For example: "I greatly enjoy reading books" means that the speaker really likes reading books a lot.
The word "great-grandfathers" refers to the grandfathers of one's grandparents. In other words, it is the generation of one's grandfathers' fathers. It is a term used to describe the paternal ancestors of one's parents, which is three generations prior to oneself.