A studious person spends a lot of their time reading and studying, and does it by choice — it is part of who they are, not just what they must do this week. So it describes a habit and a character, not a single afternoon. You would not say 'I was studious yesterday'; you say 'she has always been studious'.

The feeling of the word is mostly warm but slightly distant. It praises the person's seriousness, yet it also paints them as quiet, indoors, a little apart from noisy life. Novels use it that way constantly: the studious younger brother, the studious girl at the back of the class. Because of that shading, calling someone studious can be gentle teasing among friends, though it is never an insult.

It is a somewhat formal and written word. In real speech, most people say 'she studies a lot' or 'he's a hard worker'. You are likelier to read studious in a school report, a book, or a description of someone's character than to hear it in conversation.

It can also describe things that show that quality — a studious expression, a studious silence — meaning they look like the face or the quiet of someone deep in a book.

There is a second, less common use worth recognising: 'studious' sometimes means deliberate and careful, as in 'a studious avoidance of the subject'. That use is closer to 'studied', and you meet it mostly in formal writing.