Abstract nouns (e.g. data, energy, quality, problem ...) are nouns which are difficult to visualise in the same way as concrete nouns (e.g. tree, house, plate, flower). Abstract nouns indicate things which exist as ideas, feelings, topics, states, actions, problems, and so on. Even the words we use to describe abstract nouns are themselves abstract nouns. So we seem to be going round in circles. How do we pin down the meaning? This is a problem for dictionary makers, too; how to give a good definition of something which is abstract? Words normally exist in context and the context is where we normally get our meaning. Lexicographers take a word out of normal context to create a definition. Then to make things clearer they put a bit of context back in the form of examples.
Are abstract nouns more difficult to learn than concrete nouns? Perhaps not. If you are a learner of English as another language, you already have these notions in your first language. So perhaps that's not so problematic. What may be more difficult is how these words are used in texts and what they mean or refer to in context. Some of these nouns are very general and could apply to almost anything: thing, reason, effect, fact, problem, importance, etc. When you see these words in a text you have to look elsewhere in the text for the meaning. Usually you find it somewhere before the abstract word (anaphorically) but occasionally you might find is later (cataphorically).
There is a category of general nouns which are used in this way which has been referred to by a number of names: General nouns (Halliday and Hasan, 1976); anaphoric nouns (Francis, 1986); Catch-all nouns; Carrier nouns (Ivanic, 1991); and shell nouns (Schmid, 2000). Anaphoric nouns implies that they are only used anaphorically, which is not the case (although the vast majority probably are anaphoric). Shell nouns now seems to be the preferred usage. The following two examples show how shell nouns are used in texts, both anaphorically (backward pointing) and cataphorically (forward pointing)
The following example is from an article entitled "Did prehistoric women hunt? New research suggests so" by Annemieke Milks, published in The Conversation and you can read the full article here. It is an example of anaphoric (backward pointing) reference.
The article has more examples of shell noun use, so click on the link above to see how they are used in this text. You can find: "The research", "the same period", "This idea", "This finding", "This model".
The next example is from an article entitled "Greenland is melting: we need to worry about what’s happening on the largest island in the world" by Jonathan Bamber, published in The Conversation and you can read the full article here. It is an example of cataphoric (forward pointing) reference.
The article also has more examples of shell noun use (all anaphoric), so click on the link above to see how they are used in this text. You can find: "this problem", "the approach", "These scenarios", "This scenario", "These models".
The Academic Word List contains many abstract nouns which could be used as shell nouns and you can download a pdf of AWL Shell Nouns. You can see how some of them are used on the Anaphoric Nouns page. You can also learn about the collocates of many of these nouns in the Collocation Game.