Studiousness is the quality of being studious — the habit of spending a lot of time reading and studying, treated as a thing you can talk about on its own. Studious describes a person; studiousness names the trait inside them. Because it names a quality rather than a person, it turns up where people discuss character: school reports, essays about someone's life, descriptions of a family. 'Her studiousness impressed her teachers.' 'He was known for his quiet studiousness.' You would not use it about one homework session — it is about a lasting way of being. This is a rare and formal word, rarer than studious itself. Even educated speakers reach for something simpler in conversation: 'how hard she works', 'she's always reading'. So recognise it when you read it, but you do not need to use it. If you want to say the same thing more naturally, use the describing word: 'she is very studious'. It is uncountable, so there is no plural and no 'a studiousness' — you say 'his studiousness', 'a certain studiousness', 'great studiousness'. It can carry the same faint shading as studious: admiration for the seriousness, with a hint that the person is quiet and lives among books.

好學、用功（把「愛唸書」這個特質當成一個名詞來談）。屬於較正式、少見的書面用語，日常口語多半直接說 studious 或「他很用功」。