I see three nouns in that sentence — four, if we count the gerund.
http://guidetogrammar.org/grammar/nouns.htm
What do you see?
The noun in the sentence "Aunt Jane has an understanding of the habits of whales" is "understanding." To identify the noun in a sentence, you need to look for a word that represents a person, place, thing, or idea. In this sentence, "understanding" represents a concept or idea. It is a noun because it refers to the concept or state of comprehending or knowing something. In this case, it refers to Aunt Jane's knowledge or familiarity with the habits of whales.
