He was watching her movements attentively.

He apologized from the bottom of his heart.

He swore loyalty to his country.

This building is located in the center of the city.

His room is decorated with colorful abstract paintings.

The content of his art work expresses his own emotions and experiences.

My dog recently contracted strongylosis.

Understanding data structures is important for improving programming skills.

He ate one apple.

Abstract nouns, unlike nouns that refer to concrete things, refer to concepts or states that do not have a specific shape or color.